Find a better place for setting Layout/Paint hook related flags.
Find a better place for setting Layout/Paint hook related flags.

These hooks are supposed to be called once after a commit, not after
the WebViewImpl's root graphics layer was reset to 0.
Setting these flags in didCommitLoad looks much more sane and
straightforward. Also this fixes a subtle bug: if you don't create
the WebView with a proper size right away, and instead create it with
0x0 size, navigate and then resize, you won't get the hooks called.

BUG=529340

Review URL: https://codereview.chromium.org/1380363005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#356298}

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