Remove raster_interface from main thread context

It's no longer necessary now that GpuImageDecoderCache does not rely
on RasterInterface.

Change-Id: I914f2ece1c7ff7b8a98dbf6e2b7100be0631e228
Reviewed-on: https://chromium-review.googlesource.com/956403Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542594}

content/renderer/render_thread_impl.cc

@@ -1629,7 +1629,7 @@ RenderThreadImpl::SharedMainThreadContextProvider() {
 
   bool support_locking = false;
   bool support_gles2_interface = true;
-  bool support_raster_interface = true;
+  bool support_raster_interface = false;
   bool support_oop_rasterization = false;
   shared_main_thread_contexts_ = CreateOffscreenContext(
       std::move(gpu_channel_host), GetGpuMemoryBufferManager(),

content/renderer/webgraphicscontext3d_provider_impl.cc

@@ -96,7 +96,6 @@ cc::ImageDecodeCache* WebGraphicsContext3DProviderImpl::ImageDecodeCache() {
   // TransferCache is used only with OOP raster.
   const bool use_transfer_cache = false;
 
-  DCHECK(provider_->RasterInterface());
   image_decode_cache_ = std::make_unique<cc::GpuImageDecodeCache>(
       provider_.get(), use_transfer_cache, kN32_SkColorType,
       kMaxWorkingSetBytes);