Change type of |upload_pixels_| to improve performance

For issue 817341, fuzzer tests were taking a long time due to calls to
std::vector<uint8_t>::resize(), as it initializes each element one by
one. Switching to std::unique_ptr<uint8_t[]> speed things up, as the
elements are not initialized. As VideoResourceUpdater allocates a
temporary buffer to hold a video frame, it could also benefit from the
improved execution time.

BUG=817341
TEST=media_unittests pass

Change-Id: I63c1afce3eb6b654c8863a555642ee3e05c1c883
Reviewed-on: https://chromium-review.googlesource.com/1137063
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575479}
2 files changed