Chromium crashes under DynamoRIO

Using Chromium browser Version 53.0.2785.143 Built on Ubuntu , running on Ubuntu 16.04 (64-bit) I get the following error message:

```
./drrun -disable_traces -no_enable_reset -stack_size 100K -- /usr/lib/chromium-browser/chromium-browser

<(1+x) Handling our fault in a TRY at 0x000055d7bc7f15b6>

<-- parent 50689 forked child 50693 -->

[50689:50689:1129/212945:FATAL:zygote_host_impl_linux.cc(182)] Check failed: ReceiveFixedMessage(fds[0], kZygoteBootMessage, sizeof(kZygoteBootMessage), &boot_pid).
#0 0x7f37f7e1e6ae base::debug::StackTrace::StackTrace()
#1 0x7f37f7e3b70e logging::LogMessage::~LogMessage()
#2 0x7f37f61d906b content::ZygoteHostImpl::LaunchZygote()
#3 0x7f37f61d7fc7 content::ZygoteCommunication::Init()
#4 0x7f37f61d85be content::CreateZygote()
#5 0x7f37f5e91abc content::BrowserMainLoop::EarlyInitialization()
#6 0x7f37f5e95051
#7 0x7f37f5e8d863 content::BrowserMain()
#8 0x7f37f5dfdf4d
#9 0x7f37f5dfd141 content::ContentMain()
#10 0x7f37f88c219a ChromeMain
#11 0x7f37ec901830 __libc_start_main
#12 0x7f37f88c2049 _start

0x00000000000002d8
0x3231313a39383630
>

Aborted (core dumped)
```

You will notice that `-disable_traces`, `-enable_no_reset` and increased stack size does not seem to help.

Start by reproducing the reported command with drrun and Chromium on Ubuntu 16.04, then inspect the DynamoRIO debug check at core/dispatch.c:141 and the surrounding fork and thread-handling path. Done means identifying and fixing the cause of the dcontext assertion, with the Chromium launch succeeding under the reported options and a regression test or documented reproduction path.

Written by the indexing model from the issue text.