Implement autoplay gesture override experiment.

LayoutTests/media/video-autoplay-experiment-expected.txt

Index: LayoutTests/media/video-autoplay-experiment-expected.txt
diff --git a/LayoutTests/media/video-autoplay-experiment-expected.txt b/LayoutTests/media/video-autoplay-experiment-expected.txt
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+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+CONSOLE WARNING: Failed to execute 'play' on 'HTMLMediaElement': API can only be initiated by a user gesture.
+END OF TEST
+  Check if the autoplay gesture override experiment works.  There are a lot
+  of config options, so this test just runs all of them.
+
+  The "results" table contains one row per config tested.
+  == Test Inputs ==
+  #      - config number, in case you'd like to run just one.
+  Flags  - autoplay experiment setting being tested.
+               E      - "enabled" in command line.
+               V      - "-ifvisible"
+               M      - "-ifmuted"
+               p      - "-playmuted"
+               For example, EVM means "enabled-ifvisible-ifmuted".
+               This test does not check -ifmobile since that check always
+               fails outside of android.
+  Play w/- how is play requested?
+               none   - play is not requested.
+               attr   - autoplay attribute is set on the element.
+               play() - play() called after media is ready to play.
+  Mute   - how is media muted?
+               none - media is not muted.
+               attr - muted attribute is set on the element.
+               js   - muted property is set after media is ready to play.
+  Vis    - is media visible?
+               onscreen  - element starts out onscreen.
+               scroll    - element starts offscreen, scrolled into view once
+                           it is ready to play.
+               offscreen - element starts out offscreen and stays offscreen.
+
+  == Test Outputs ==
+  Early?  - did playback start before element was scrolled onscreen?  For
+            tests in which Vis!=scroll, this is reported as "-".
+  Played? - did playback start by the conclusion of the test?
+  Muted?  - was the media muted?  If the media didn't play, then this is
+            reported as "-".
+
+#	Flags	Play w/	Mute	Vis	Early?	Played?	Muted?
+0		none	none	onscree	-	no	-
+1	E	none	none	onscree	-	no	-
+2	EV	none	none	onscree	-	no	-
+3	EVM	none	none	onscree	-	no	-
+4	EVp	none	none	onscree	-	no	-
+5		attr	none	onscree	-	no	-
+6	E	attr	none	onscree	-	played	unmuted
+7	EV	attr	none	onscree	-	played	unmuted
+8	EVM	attr	none	onscree	-	no	-
+9	EVp	attr	none	onscree	-	played	muted
+10		play()	none	onscree	-	no	-
+11	E	play()	none	onscree	-	played	unmuted
+12	EV	play()	none	onscree	-	played	unmuted
+13	EVM	play()	none	onscree	-	no	-
+14	EVp	play()	none	onscree	-	played	muted
+18	EVM	none	attr	onscree	-	no	-
+23	EVM	attr	attr	onscree	-	played	muted
+28	EVM	play()	attr	onscree	-	played	muted
+33	EVM	none	js	onscree	-	no	-
+38	EVM	attr	js	onscree	-	played	muted
+43	EVM	play()	js	onscree	-	played	muted
+50		attr	none	scroll	no	no	-
+51	E	attr	none	scroll	yes	played	unmuted
+52	EV	attr	none	scroll	no	played	unmuted
+54	EVp	attr	none	scroll	no	played	muted
+55		play()	none	scroll	no	no	-
+56	E	play()	none	scroll	yes	played	unmuted
+57	EV	play()	none	scroll	no	played	unmuted
+59	EVp	play()	none	scroll	no	played	muted
+95		attr	none	offscre	-	no	-
+96	E	attr	none	offscre	-	played	unmuted
+97	EV	attr	none	offscre	-	no	-
+99	EVp	attr	none	offscre	-	no	-
+