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Currently, an app can exceed the number of decoders that vendor backends
support (for example, by creating multiple video elements), leading to
unpredictable behaviour including crashes.  While most apps don't do
this, we have seen some problems crop up e.g. when ads are injected into
an existing page.

For now, restrict to 1 video decoder and 2 audio decoders.

BUG=internal b/26910920

Review-Url: https://codereview.chromium.org/2173593002
Cr-Commit-Position: refs/heads/master@{#407215}