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: http://src.chromium.org/svn/trunk/src/build@239451 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1715,6 +1715,10 @@
       ['enable_extensions==1', {
         'grit_defines': ['-D', 'enable_extensions'],
       }],
+      ['enable_plugins!=0', {
+
+        'grit_defines': ['-D', 'enable_plugins'],
+      }],
       ['enable_printing!=0', {
         'grit_defines': ['-D', 'enable_printing'],
       }],