Flatbuffers: Ignore exit time destructor warning.

DefaultAllocator::instance() method in flatbuffers.h defines a static object,
leading to a destructor being run at exit-time. However, many targets in Chrome
use the "wexit_time_destructors" compiler config, which generates warnings when
exit time destructors are generated. This causes compile errors, when 
flatbuffers.h is transitively included from a file in one of such targets. To
fix this, suppress the exit time destructor warning as part of the flatbuffer
config.

BUG=737847

Change-Id: I706be9e8c204ed24a82aa04b33b00a78cf36489d
Reviewed-on: https://chromium-review.googlesource.com/554318
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484403}

third_party/flatbuffers/BUILD.gn

@@ -7,6 +7,10 @@ import("//third_party/flatbuffers/flatbuffer.gni")
 
 config("flatbuffers_config") {
   include_dirs = [ "src/include" ]
+
+  if (is_clang) {
+    cflags = [ "-Wno-exit-time-destructors" ]
+  }
 }
 
 # The part of FlatBuffers that Chrome is interested in.