Handle null return value for getLastResortFallbackFont
Handle null return value for getLastResortFallbackFont

getNonRetainedLastResortFallbackFont can, despite the name which seems
to suggest otherwise, return 0. Handle this case by bailing out and
asserting.

BUG=341628
R=dglazkov@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166678

Review URL: https://codereview.chromium.org/150103014

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@166740 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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