Fix crash in CSS.getLayoutTreeAndStyles

For pages where there was a FrameOwnerElement with a null
ContentDocument CSS.getLayoutTreeAndStyles would crash.  This patch
fixes that.

BUG=680497, 546953

Review-Url: https://codereview.chromium.org/2623273006
Cr-Commit-Position: refs/heads/master@{#443513}
1 file changed