Add null checks in CanvasResourceProvider::Create

Null ptr checks were missing on context_provider_wrapper, resulting
in crashes after a GPU context loss.

BUG=863752

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I4b6f1fc394deb3f5567fee1da6f74b66aa7db79d
Reviewed-on: https://chromium-review.googlesource.com/1140282Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575674}

third_party/blink/renderer/platform/graphics/canvas_resource_provider.cc

@@ -420,6 +420,8 @@ std::unique_ptr<CanvasResourceProvider> CanvasResourceProvider::Create(
         if (presentation_mode !=
             CanvasResourceProvider::kAllowImageChromiumPresentationMode)
           continue;
+        if (!context_provider_wrapper)
+          continue;
         if (!gpu::IsImageFromGpuMemoryBufferFormatSupported(
                 color_params.GetBufferFormat(),
                 context_provider_wrapper->ContextProvider()
@@ -454,6 +456,8 @@ std::unique_ptr<CanvasResourceProvider> CanvasResourceProvider::Create(
             size, color_params, resource_dispatcher);
         break;
       case kTextureResourceType:
+        if (!context_provider_wrapper)
+          continue;
         provider = std::make_unique<CanvasResourceProviderTexture>(
             size, msaa_sample_count, color_params, context_provider_wrapper,
             resource_dispatcher);