Remove PNaCl's RequestFirstInstall, use resource throttle instead

This removes PNaCl's RequestFirstInstall and its use of
the component updater OnDemand API. A different CL:
https://codereview.chromium.org/25713007/
will do the OnDemand calls internally.

This removes the async IPCs, callbacks, and observers that
were used to support the old code. The new code just
throttles/blocks the pexe URL request until OnDemand
installation is done.

Reverts: https://codereview.chromium.org/19863003/, plus
other CLs.

BUG=none
R=cpu@chromium.org, dmichael@chromium.org
TBR=jln

Review URL: https://codereview.chromium.org/55463002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232827 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232827