Problems showing pdf

I have problems trying to show a pdf, in android directly it never loads them and in Ios, it depends on the server, if it is https it shows them, if it is http, it does not show them.

In apple I have no feedback.
In Android I have this the first time I run the plugin:

```
I/WebViewFactory(32731): Loading com.google.android.webview version 68.0.3440.85 (code 344008500)
I/art (32731): Rejecting re-init on previously-failed class java.lang.Class
I/art (32731): Rejecting re-init on previously-failed class java.lang.Class
I/art (32731): Rejecting re-init on previously-failed class java.lang.Class
I/art (32731): Rejecting re-init on previously-failed class java.lang.Class
I/cr_LibraryLoader(32731): Time to load native libraries: 4 ms (timestamps 3006-3010)
E/cr_VariationsUtils(32731): Failed reading seed file "/data/user/0/com.example.webviewtest/app_webview/variations_seed_new": /data/user/0/com.example.webviewtest/app_webview/variations_seed_new: open failed: ENOENT (No such file or directory)
I/chromium(32731): [INFO:library_loader_hooks.cc(36)] Chromium logging enabled: level = 0, default verbosity = 0
I/cr_LibraryLoader(32731): Expected native library version number "68.0.3440.85", actual native library version number "68.0.3440.85"
I/cr_BrowserStartup(32731): Initializing chromium process, singleProcess=true
I/cr_base (32731): Android Locale: es_US requires .pak files: []
I/chromium(32731): [INFO:aw_field_trial_creator.cc(52)] First-WebView-Experiment found, group=Enabled
I/art (32731): Rejecting re-init on previously-failed class java.lang.Class
I/art (32731): Rejecting re-init on previously-failed class java.lang.Class
I/art (32731): Rejecting re-init on previously-failed class java.lang.Class
I/art (32731): Rejecting re-init on previously-failed class java.lang.Class
E/libEGL (32731): validate_display:255 error 3008 (EGL_BAD_DISPLAY)
W/cr_media(32731): Requires BLUETOOTH permission
W/cr_CrashFileManager(32731): /data/user/0/com.example.webviewtest/cache/WebView/Crash Reports does not exist or is not a directory
W/VideoCapabilities(32731): Unrecognized profile 2130706433 for video/avc
W/VideoCapabilities(32731): Unrecognized profile/level 0/3 for video/mpeg2
W/VideoCapabilities(32731): Unrecognized profile/level 0/0 for video/mpeg2
W/VideoCapabilities(32731): Unrecognized profile/level 0/2 for video/mpeg2
W/VideoCapabilities(32731): Unrecognized profile/level 0/3 for video/mpeg2
I/VideoCapabilities(32731): Unsupported profile 4 for video/mp4v-es
```

I'm using WebviewScaffold, but I just realized that when I try to open a fullscreen or a rect, it does not do anything (android). It also does not show any error.

Pdf examples:
http://www.pdfpdf.com/samples/Sample1.PDF (not work in android or ios)
https://www.adobe.com/support/products/enterprise/knowledgecenter/media/c4611_sample_explain.pdf (only work in ios)

Tested in Iphone 6 (ios 11.4.1), and MotoG3 (android 6.0)

This is my flutter doctor.

```
[flutter] flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel master, v0.5.8-pre.199, on Mac OS X 10.13.6 17G65, locale en-CA)
[✓] Android toolchain - develop for Android devices (Android SDK 27.0.3)
[✓] iOS toolchain - develop for iOS devices (Xcode 9.4.1)
[✓] Android Studio (version 3.1)
[✓] VS Code (version 1.25.1)
[✓] Connected devices (2 available)
```

首先，在列出的 Android 和 iOS 版本上通过 WebviewScaffold 重现报告的 PDF URL，然后检查 plugin 针对各平台的 WebView 处理。支持的 PDF URL 能够稳定加载，或平台限制已连同有用的错误反馈记录在案，即表示完成。

由索引模型根据 Issue 内容生成。