[ios blink] Fix BELayerHierarchy dealloc race

ImageTransportSurfaceOverlayMacEGL destructor transfers ownership of the
BELayerHierarchy to a local variable which is then used in a dispatch
block which also takes a strong reference on the BELayerHierarchy. This
means there's a race between the destructor and the dispatch block over
who drops the last reference to the BELayerHierarchy and calls dealloc.

Capture and retain the BELayerHierarchy in the block before dropping the
local reference. Do this before dispatch_async() to avoid a dealloc race
between the block and the local variable.

Cq-Include-Trybots: luci.chromium.try:ios-blink-dbg-fyi
Change-Id: Id96986c33bbe8c0ce52567f5314743f1bcbae3d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6435417
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1445369}