Add the pdf plugin's source in src\pdf.
I've updated gypi files to not use internal_pdf variable anymore, which was brought in from pdf repo's supplemental.gypi.
R=thestig@chromium.org
TBR=darin
Review URL: https://codereview.chromium.org/294793003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271531 0039d316-1c4b-4281-b951-d872f2087c98