Add UMA to track software fallback in VTVideoDecodeAccelerator.

VideoToolbox can fall back to software internally for two reasons:
  1) The resolution is below the minimum (or other format incompatibility). Eventually we want Chromium to handle
fallback in this case.
  2) The sandbox was not initialized properly. This we want to know about and fix.

This CL does not distinguish the two cases.

BUG=133828

Review URL: https://codereview.chromium.org/773673002

Cr-Commit-Position: refs/heads/master@{#306692}
4 files changed