Start out deferring commits in WebViewImpl.

This is analogous to the call in WebViewImpl::setRootGraphicsLayer when deleting
the root graphics layer (as a proxy for detecting page navigation), but for when
starting up a new WebViewImpl.

BUG=470669

Review-Url: https://codereview.chromium.org/1934513004
Cr-Commit-Position: refs/heads/master@{#390865}
1 file changed