DevTools: Create PowerSaveBlocker only on Android

BUG=311608

Review URL: https://codereview.chromium.org/46933002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231331 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231331
Add WaitForInitialUserActivity policy

This CL adds a user policy that controls whether power management delays
and the session length limit should only start running after initial user
activity in a session. The SessionLengthLimiter is modified to obey the
new policy. Power management will hooked up to the new policy in a
follow-up CL.

It would technically be possible to add separate policies that determine
whether power management delays on the one hand and the session length
limit on the other hand should wait for initial user activity. However,
it does not make sense for one of these to start running on session start
while the other waits for user activity. Therefore, a single policy is
added for both.

BUG=310176
TEST=Updated unit and browser tests

Review URL: https://codereview.chromium.org/35943002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231332 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231332