Merge 62261, 62267, 62265 & 62679...

Merge 62261, 62267, 62265 & 62679
Merge 62261 - This changes GetPluginInfo so that we can request a list
of plugins that match the criterion (mime type/url) instead of just
the first one that matches.

This is in preparation for implementing a way for us to switch which
flash plugin we select based on a whitelist of domains.

Review URL: http://codereview.chromium.org/3530017

Merge 62679 - This adds a plugin selection policy for selecting which
plugin is allowed for a particular domain.

It is only used on ChromeOS.  It reads from a file that is installed
in a known location on ChromeOS, and uses that as it's policy.

When there are multiple plugins supporting the same mime-type, the
appropriate plugin file to load is now selected based on policy.

Review URL: http://codereview.chromium.org/3717005

Merge 62267 - Take out bogus DCHECK

This DCHECK is valid only within the browser itself -- it doesn't work
for tests, etc, which have different default plugins.

Merge 62265 - This makes it so we only add the default plugin if it's enabled.

BUG=http://crosbug.com/7403
TEST=ran chrome unit_tests, ran chromeos version of chrome, passed trybots.
TBR=gspencer@chromium.org


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=62964