Revert "Removing CPU fallback in webgl::getImage"

This reverts commit 69917e2b.

Reason for revert: Removing the fallback broke Android tests.

Original change's description:
> Removing CPU fallback in webgl::getImage
> 
> According to comment below in line 787, this method
> expects the canvas resource provider to provide
> always GPU resources.
> 
> This also currently passes all the existing tests,
> so it's probably safe to remove this fallback.
> 
> Bug: 1035589
> Change-Id: I82a426d29a8a37755ee36d2a14f006bd62fa5bb3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225479
> Reviewed-by: James Darpinian <jdarpinian@chromium.org>
> Commit-Queue: Juanmi Huertas <juanmihd@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#774683}

TBR=jdarpinian@chromium.org,juanmihd@chromium.org

Change-Id: I6dcffe3cbeb3389509f4b4abcd14f3e681a6357a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1035589, 1090962
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227884Reviewed-by: Juanmi Huertas <juanmihd@chromium.org>
Commit-Queue: Juanmi Huertas <juanmihd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774771}

third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc

@@ -780,6 +780,11 @@ scoped_refptr<StaticBitmapImage> WebGLRenderingContextBase::GetImage(
           GetDrawingBuffer()->FilterQuality(), color_params,
           is_origin_top_left_, CanvasResourceProvider::RasterMode::kGPU,
           0u /*shared_image_usage_flags*/);
+  // todo(bug 1035589) Check if this cpu fallback is really needed here
+  if (!resource_provider || !resource_provider->IsValid()) {
+    resource_provider = CanvasResourceProvider::CreateBitmapProvider(
+        size, GetDrawingBuffer()->FilterQuality(), color_params);
+  }
 
   if (!resource_provider || !resource_provider->IsValid())
     return nullptr;