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Similarly to chrome://drive-internals, this patch introduce a webui page
showing what's going on in the system.

Currently, only mounted file systems, and number of active requests are shown,
however more is coming soon.

TEST=Tested manually by invoking chrome://fsp-internals with a running
     providing extension.
BUG=376095

Review URL: https://codereview.chromium.org/296003012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272886 0039d316-1c4b-4281-b951-d872f2087c98