Fix a crash caused by track insertion after load().

This patch fixes a crash caused by stale LoadMediaResource flag in
m_pendingActionFlags when load() is explicitly called on a media
element. The insertion of a <track> element triggers the crash by
triggering the scheduling of the m_loadTimer, which ends up using the
stale flag data when the timer fires. The fix is to clear the
LoadMediaResource flag from m_pendingActionFlags when a new load is
initiated.

BUG=356352
TEST=LayoutTests/media/track/track-insert-after-load-crash.html

Review URL: https://codereview.chromium.org/211373009

git-svn-id: svn://svn.chromium.org/blink/trunk@170003 bbb929c8-8fbe-4397-9dbb-9b2b20218538