Always notify the MediaPlayer of any seek.

This change removes short circuit logic for seeking to the current position.
There are certain situations, like seeking to a truncated duration, where the
HTMLMediaElement is not in the best position to determine whether a call to
the MediaPlayer is necessary or not. This change allows the
MediaPlayer/WebMediaPlayer implementation to determine what the best course
of action should be. This also makes the seeking behavior more spec
compliant because even a seek to the current position should have the
seeking attribute be true until a stable state occurs. This is guaranteed
when calling into the MediaPlayer and allows all seeks to be treated the
same at the HTMLMediaElement level.

BUG=266631
TEST=LayoutTests/media/video-seek-to-current-position.html

Review URL: https://codereview.chromium.org/431903003

git-svn-id: svn://svn.chromium.org/blink/trunk@179347 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Mark link-rel-prefetch-and-subresource flaky

TBR=alancutter@chromium.org
NOTRY=true
BUG=399459

Review URL: https://codereview.chromium.org/431153003

git-svn-id: svn://svn.chromium.org/blink/trunk@179348 bbb929c8-8fbe-4397-9dbb-9b2b20218538