Add the pdf plugin's source in src\pdf.

build/common.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 1257 matching lines @@
     'disable_fatal_linker_warnings%': 0,
 
     'release_valgrind_build%': 0,
 
     # TODO(thakis): Make this a blacklist instead, http://crbug.com/101600
     'enable_wexit_time_destructors%': 0,
 
     # Build libpeerconnection as a static library by default.
     'libpeer_target_type%': 'static_library',
 
-    # Set to 1 to compile with the built in pdf viewer.
-    'internal_pdf%': 0,
-
     # Set to 1 to compile with the OpenGL ES 2.0 conformance tests.
     'internal_gles2_conform_tests%': 0,
 
     # Set to 1 to compile the filter fuzzer.
     'internal_filter_fuzzer%': 0,
 
     # NOTE: When these end up in the Mac bundle, we need to replace '-' for '_'
     # so Cocoa is happy (http://crbug.com/20441).
     'locales': [
       'am', 'ar', 'bg', 'bn', 'ca', 'cs', 'da', 'de', 'el', 'en-GB',
@@ skipping 4038 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }