Directly access the std::vector buffer backing store to write bytes. By
reserving enough space upfront we can avoid many superflous bounds checks
that were previously necessary when using push_back.

Change-Id: I9b2fb467809e40743b0d9409c9cccb0c6f36f8c1
Reviewed-on: https://chromium-review.googlesource.com/c/1425910
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59036}