[PE] Speculative fix for crash called from PageOverlay::Update

Don't know why the parent GraphicsLayer is null on Mac only.
The crash spiked since M67, but on M66 the non-SlimmingPaintV175 path
also crashed for the same reason.

Bug: 848442
Change-Id: I34ed17f8aaa193128f295b1cb6139a51164365e8
Reviewed-on: https://chromium-review.googlesource.com/1104697
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568061}
1 file changed