Add a PDF component extension

This adds a PDF component extension which is used for rendering PDFs with an
out of process PDF plugin. It adds the extension as a MIME handler for pdf file
types. The extension is currently only added if the --out-of-process-pdf switch
is passed to Chrome, AND the PDF plugin is available.

BUG=303491

Review URL: https://codereview.chromium.org/96233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239451 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239451