Propagate kReportVp9AsAnUnsupportedMimeType flag to renderer

Problem: Setting switches::kReportVp9AsAnUnsupportedMimeType command
line flag on the Main process has no affect, as it is not
white-listed to be propagated to the render process in:
RenderProcessHostImpl::PropagateBrowserCommandLineToRenderer()

Solution: Add switches::kReportVp9AsAnUnsupportedMimeType to white-list

Bug: None
Change-Id: I037edfd0457d7ae87743854b8fd7d2d92e7f5107
Reviewed-on: https://chromium-review.googlesource.com/981392Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546136}

content/browser/renderer_host/render_process_host_impl.cc

@@ -2647,6 +2647,7 @@ void RenderProcessHostImpl::PropagateBrowserCommandLineToRenderer(
     switches::kReducedReferrerGranularity,
     switches::kRegisterPepperPlugins,
     switches::kRendererStartupDialog,
+    switches::kReportVp9AsAnUnsupportedMimeType,
     switches::kSamplingHeapProfiler,
     switches::kShowPaintRects,
     switches::kStatsCollectionController,