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IsStorageTypeMappable() was defined and used in video_frame.cc only.
Export it as VideoFrame::IsStorageTypeMappable() to use in other
places.

BUG=None
TEST=Pass CQ.

Change-Id: Ib3bcf5291e64464c8a75f0e9c5fe2f4c62b179d5
Reviewed-on: https://chromium-review.googlesource.com/c/1356459Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Shuo-Peng Liao <deanliao@google.com>
Cr-Commit-Position: refs/heads/master@{#613010}