Experimental flag for HiDPI-aware PDF Viewer

Adds a flag --enable-hidpi-pdf-plugin so HiDPI rendering in the PDF Viewer can
be controlled by this flag.

Add a pepper priv interface to query the flag.

BUG=114674
TEST=Manual

Review URL: https://chromiumcodereview.appspot.com/10802004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148567 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed