Check for null scrollingLayer when getting nonFastScrollableRects
Check for null scrollingLayer when getting nonFastScrollableRects

Page::nonFastScrollableRects gets the non fast scrolling regions from the
scrolling layer. ClusterFuzz found this method will crash if executed before
a scrolling layer exists so I added a NULL check here. Note that this method
is only ever called from Internals.

BUG=511010

Review URL: https://codereview.chromium.org/1268683004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199747 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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