Remove keyframe detection from MediaPlayerBridgeTest.PresentationTimestampsDoNotDecrease

The flag in question is only meant to apply to encoders, not decoders (undocumented).  It was an unintentional that the HW decoder on Nexus5 set the flag.

BUG=325999

Review URL: https://codereview.chromium.org/106083003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239217 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239217