Some clarification on why this can be disabled.

Experiments for VrShell will require this field to be enabled, but since overlay's working correctly in VrShell ( issue 673886 ) is currently blocking the launch of VrShell (issue 641038), it is fine to disable the tests for now.  I'll add a note to the VrShell overlay  issue 673886  about making sure to re-enable these when that is fixed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7345d7850f8c96020a2a43792b699b70093e9ac4

commit 7345d7850f8c96020a2a43792b699b70093e9ac4
Author: liberato <liberato@chromium.org>
Date: Wed Jan 04 00:54:13 2017

Turn off VrShell experiment in testing config.

Don't enable VrShell experiment in non-channel builds, since it
turns off overlay support for video playback on Android.  Enabling
tests is blocked on supporting overlay video properly anyway, so
nothing depends on this default.

BUG= 678018 

Review-Url: https://codereview.chromium.org/2613543002
Cr-Commit-Position: refs/heads/master@{#441272}

[modify] https://crrev.com/7345d7850f8c96020a2a43792b699b70093e9ac4/testing/variations/fieldtrial_testing_config.json