Reserve fd 5 on Android and bump the base fd to 6.

BUG=611470

Review-Url: https://codereview.chromium.org/1998893002
Cr-Commit-Position: refs/heads/master@{#395518}

base/posix/global_descriptors.h

@@ -57,7 +57,12 @@ class BASE_EXPORT GlobalDescriptors {
 #else
   // 3 used by __android_log_write().
   // 4 used by... something important on Android M.
-  static const int kBaseDescriptor = 5;
+  // 5 used by... something important on Android L... on low-end devices.
+  // TODO(amistry): An Android, this mechanism is only used for tests since the
+  // content child launcher spawns a process by creating a new Activity using
+  // the Android APIs. For tests, come up with a way that doesn't require using
+  // a pre-defined fd.
+  static const int kBaseDescriptor = 6;
 #endif
 
   // Return the singleton instance of GlobalDescriptors.