Switch from build-time to run-time flags for Project Spitzer.

Contributed by dalecurtis@chromium.org.

This puts the desktop media playback pipeline behind a runtime flag
for Android; previously this was a combination of build-time and
run-time flags to avoid introducing a binary size increase before
we were ready for experiments.

This change will result in a ~480kb apk size increase on Android that has been
approved via Chrome Eng Review and klobag@ from the Chrome for Android team.
http://crbug.com/570711 tracks this size increase and plans to reduce it in
the future.

It paves the way for unifying our playback stacks across platforms and brings
previously-missing features and improved security to Android playback. More
technical details can be found in the linked bug below and design doc:
https://goo.gl/qC3OuL

BUG=507834, 570711, 570762
TEST=builds with gn/gyp work with unified path.
TBR=grt@chromium.org,dpranke@chromium.org,liberato@chromium.org,creis@chromium.org

Review URL: https://codereview.chromium.org/1541513002

Cr-Commit-Position: refs/heads/master@{#366208}
16 files changed