android-首次在native页面调用driver.contexts，会导致之后异常

* **Macaca Version**:2.0.9

* **Macaca Doctor Logs**:
macaca-doctor version: 2.0.2

Node.js checklist:

node env: /usr/local/bin/node
node version: v6.10.2

iOS checklist:

Xcode Command Line Tools is ready, version: 2347.
xcodebuild version: 8.3.3
iproxy[usbmuxd] is installed at: `/usr/local/bin/iproxy`
ios_webkit_debug_proxy is installed at: `/usr/local/bin/ios_webkit_debug_proxy`
carthage is installed, version: 0.23.0

Android checklist:

JAVA version is `1.8.0_121`
JAVA_HOME is set to `/Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home`
ANDROID_HOME is set to `/Users/joko/Library/Android/sdk`
Platforms is set to `/Users/joko/Library/Android/sdk/platforms/android-25`
ADB tool is set to `/Users/joko/Library/Android/sdk/platform-tools/adb`
gradle is installed, version: 3.5

Installed driver list:

android: 2.0.36
electron: 1.1.6 [out-of-date]
ios: 2.0.23 [out-of-date]

* **Operate System**:
mac os 10.12.5

* **Programming Language**: python 2.7

预置环境:android 5.0 webview版本：54.0.2840.85
步骤：
1.使用macaca进行测试，停留在native界面调用driver.contexts方法，报错消息
```log
Error: None
Message: no such session
(Driver info: chromedriver=2.27.440174 (e97a722caafc2d3a8b807ee115bfb307f7d2cfd9),platform=Mac OS X 10.12.5 x86_64)
```
2.此时进入native或者webview页面调用driver.contexts，均会报以上错误
3.如果首次在webview页面调用driver.contexts方法，可正常打印上下文数据，之后再进入任何页面，均可正常打印
```log
刷新页面
[u'NATIVE_APP', u'CDwindow-C16FA52D-6D9C-A6BC-7723-B8141C0F49CF']
刷新页面
[u'NATIVE_APP']
刷新页面
[u'NATIVE_APP', u'CDwindow-7809B59E-5612-1030-BC4A-B6F3768BF95D']
刷新页面
```

问题描述：首次调用场景会导致driver.contexts之后的异常，请帮忙修复此问题