Move deferred commit logic from WebViewImpl to Document.
This is more natural here, and will make it easy to add a bit that
tracks if a document would be in "deferred commits" mode even if its
not the main document. Then we can use that bit to trigger pipeline
throttling for iframes when they're loading avoiding FOUC in a sane
way.
This patch doesn't change any behavior and only moves the logic.
BUG=521692
TEST=DocumentLoadingRenderingTest.*
Review URL: https://codereview.chromium.org/1922793002
Cr-Commit-Position: refs/heads/master@{#389829}
14 files changed
