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This adds a base::Feature, kPreloadMetadataSuspend, for suspending
preload=metadata players that are either audio-only or have video
and have a poster image.

Given the uncertainty and issues with canplay, canplaythrough, the
current version of this feature signals HAVE_ENOUGH when after a
suspended startup successfull completes.

This also fixes a minor issue where streams with a positive start
time were not suspending at the right time.

BUG=694855
TEST=enabling feature still passes all bots. More layout tests,
which are disabled by default in this CL since the feature is
disabled by default for now.

Change-Id: Iccd79eddbd7a3b03933b3090eb99ad19c2fed768
Reviewed-on: https://chromium-review.googlesource.com/978867Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545817}