Add assert that updateLayoutTree clears dirty bits.
Add assert that updateLayoutTree clears dirty bits.

We should never leave any dirty bits for style invalidation or style recalc
after having run updateLayoutTree. This adds a tree walk that makes sure all
bits are cleared.

BUG=492730

Review URL: https://codereview.chromium.org/1156273002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@197435 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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