H264SPS: Centralize computation of coded size and visible rect.

These new methods are paranoid about undefined behavior, and they
replace similar code in H264Decoder and EsParserH264.

BUG=639128

Review-Url: https://codereview.chromium.org/2268183009
Cr-Commit-Position: refs/heads/master@{#415488}
7 files changed