Add exceptions for gfx/gpu interfaces used by WebVR

The WebKit/Source/modules/vr/ code uses gfx::GpuMemoryBufferHandle and
gpu::MailboxHolder for transferring frame images, these symbols
shouldn't block presubmit checks.

BUG=763980

Change-Id: I2f9fdd173f0173bdb51337d3943b590f42550528
Reviewed-on: https://chromium-review.googlesource.com/804742
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521636}

third_party/WebKit/Tools/Scripts/audit-non-blink-usage.py

@@ -116,7 +116,13 @@ _CONFIG = [
             'third_party/WebKit/Source/modules/vr/',
             'third_party/WebKit/Source/modules/webgl/',
         ],
-        'allowed': ['gpu::gles2::GLES2Interface'],
+        # These modules need access to GL drawing, cross-process image
+        # transport, and related interfaces.
+        'allowed': [
+            'gfx::GpuMemoryBufferHandle',
+            'gpu::gles2::GLES2Interface',
+            'gpu::MailboxHolder',
+        ],
     },
     {
         'paths': [