Fix Android video representation lock

VideoImageReaderImageBacking::SkiaGraphiteDawnImageRepresentation was
accessing the VideoImageReadImageBacking::stream_texture_sii_ without
first taking the lock. The representation did take the lock before using
the pointer, but the pointed to object may have been destroyed before
the lock is acquired. This is a speculative fix for a graphite crash in
BeginAccess().

Bug: 378913956
Change-Id: I63ccc1742fcaaf8f0ceceb4821016f6209d7d694
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6017672
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Kyle Charbonneau <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1383038}