[lensoverlay] Fix blur going out of bounds

On Mac, the blur layer is not forced to the bounds of the parent,
so the blur was bleeding over the edge because I forgot to change to
use local bounds on invocation in http://shortn/_xSrDcV3N21.

Change-Id: I5619a04b701fc205692132e1569d91cb50d998cd
Fixed: 391672306
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6194775
Commit-Queue: Duncan Mercer <mercerd@google.com>
Reviewed-by: Juan Mojica <juanmojica@google.com>
Cr-Commit-Position: refs/heads/main@{#1410568}