Remove gpu raster statefulness from WebViewImpl

This state only exists for unit tests, and they can just look at the
side effects on LayerTreeHost directly. This removes a widget-only
state from WebViewImpl, so we don't have to worry about setting it
correctly if it changed while the RenderWidget was frozen.

R=enne@chromium.org

Change-Id: I360da46f62f7754e4b79af33ff09f0e115051863
Bug: 912193
Reviewed-on: https://chromium-review.googlesource.com/c/1418837
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624272}
4 files changed