Fix Android multi-process tests on M.

base/posix/global_descriptors.h

Index: base/posix/global_descriptors.h
diff --git a/base/posix/global_descriptors.h b/base/posix/global_descriptors.h
index c774634f59bf9c45bd9acce2a7762de19b3b257b..1761e2518f82f36f9fd5832585d998a9c41a2531 100644
--- a/base/posix/global_descriptors.h
+++ b/base/posix/global_descriptors.h
@@ -55,7 +55,9 @@ class BASE_EXPORT GlobalDescriptors {
 #if !defined(OS_ANDROID)
   static const int kBaseDescriptor = 3;  // 0, 1, 2 are already taken.
 #else
-  static const int kBaseDescriptor = 4;  // 3 used by __android_log_write().
+  // 3 used by __android_log_write().
+  // 4 used by... something important on Android M.

[Mark Mentovai, 2016/05/04 12:07:12]

Wish we had more confidence in what this was. What if it’s a leaked FD to
something that we really don’t want access to?

[Anand Mistry (off Chromium), 2016/05/04 21:48:42]

The problem I came across is that closing or dup'ing over this fd will hang.
Even in the forked helper process which has no need to access anything beside
the stdout/stderr and the parent pipe. All I know is that this is a socket or
pipe (it's not a file). I didn't do any further digging as to what this is.

Fortunately, I don't believe this is used outside of tests. The normal Android
child process starter (in content/) doesn't have any control over fd mappings
due to all the Java that runs along side the C++ in the same process. So the
test child helper should be the only thing on Android which uses this.

+  static const int kBaseDescriptor = 5;
 #endif
 
   // Return the singleton instance of GlobalDescriptors.