Fix LayoutTests that assume canplay, playing, and canplaythrough will only fire once.

Several tests are written under the assumption that the canplay, playing,
and canplaythrough events will only ever fire once. While this is true
for the current implementation, the HTML5 spec allows these events to
fire multiple times during the life of an HTMLMediaElement. This
change just modifies the tests so that they properly reflect their
intent, which is to run some code the first time these events fire.
This frees us to change how we do readyState transitions, which cause
extra events to fire, without breaking these tests.

BUG=144683

Review URL: https://codereview.chromium.org/90173002

git-svn-id: svn://svn.chromium.org/blink/trunk@162724 bbb929c8-8fbe-4397-9dbb-9b2b20218538