Fix crash about mask layers

The return value of CompositedLayerMapping::updateChildClippingMaskLayer()
only indicates if m_childClippingMaskLayer is changed. We need to check
other conditions to update the references to it.

BUG=553045
TEST=CompositedLayerMappingTest.ClippingMaskLayer

Review URL: https://codereview.chromium.org/1427843009

Cr-Commit-Position: refs/heads/master@{#358738}
3 files changed