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to be used by pnacl for scratch files during compilation and linking.

This is in-lieu of the current use of pepper temp files, which are not
allowed in incognito mode, and are more cumbersome to clean up on surfaway.

Use this interface for the .o file in pnacl coordinator.  The .nexe still uses
pepper temp files, since we haven't moved that over for caching yet.
We will clean that up later.

Also do some crude quota enforcement for these files, using the
existing reverse service interface.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=2683
TEST= none -- pnacl compilation still works

Review URL: https://chromiumcodereview.appspot.com/10815080

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148964 0039d316-1c4b-4281-b951-d872f2087c98