Restore null checks to fix LinkHighlight crashes
Restore null checks to fix LinkHighlight crashes

LinkHighlight assumes that its paint invalidation container
is composited, but crash reports show that this isn't always the
case.

This restores the null check in
LinkHighlight::attachLinkHighlightToCompositingLayer
that was removed in http://crrev.com/757933003.

This also adds a null check to
LinkHighlight::computeHighlightLayerPathAndPosition,
effectively restoring the check removed in
http://crrev.com/809113002 (but adding a check in
the caller of mapPointToPaintBackingCoordinates rather
than in that function).

BUG=431961

Review URL: https://codereview.chromium.org/929353003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190361 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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