Add runtime flag for choosing WMPI or WMPA.

Adds a new switch, --enable-unified-media-pipeline to
control usage of WMPI vs WMPA.  Will be needed for finch
experiments as well as local testing.

BUG=507834
TEST=builds, command line flag works

Review URL: https://codereview.chromium.org/1431193003

Cr-Commit-Position: refs/heads/master@{#358695}
4 files changed