Make gtk2/gtk3 compile time switchable.

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 690 matching lines @@
       # which would lead to all code be optimized with -O2. See crbug.com/407544
       'use_lto%': 0,
       'use_lto_o2%': 0,
 
       # Allowed level of identical code folding in the gold linker.
       'gold_icf_level%': 'all',
 
       # Libxkbcommon usage.
       'use_xkbcommon%': 0,
 
+      # Whether we use GTKv3 on linux.
+      'use_gtk3%': 0,
+
       # Control Flow Integrity for virtual calls and casts.
       # See http://clang.llvm.org/docs/ControlFlowIntegrity.html
       'cfi_vptr%': 0,
       'cfi_diag%': 0,
 
       'cfi_blacklist%': '<(PRODUCT_DIR)/../../tools/cfi/blacklist.txt',
 
       # Whether the entire browser uses toolkit-views on Mac instead of Cocoa.
       'mac_views_browser%': 0,
 
@@ skipping 404 matching lines @@
     'use_udev%': '<(use_udev)',
     'os_bsd%': '<(os_bsd)',
     'os_posix%': '<(os_posix)',
     'use_dbus%': '<(use_dbus)',
     'use_glib%': '<(use_glib)',
     'use_pango%': '<(use_pango)',
     'use_cairo%': '<(use_cairo)',
     'use_ozone%': '<(use_ozone)',
     'use_ozone_evdev%': '<(use_ozone_evdev)',
     'use_xkbcommon%': '<(use_xkbcommon)',
+    'use_gtk3%': '<(use_gtk3)',
     'use_clipboard_aurax11%': '<(use_clipboard_aurax11)',
     'desktop_linux%': '<(desktop_linux)',
     'use_x11%': '<(use_x11)',
     'use_gnome_keyring%': '<(use_gnome_keyring)',
     'linux_fpic%': '<(linux_fpic)',
     'chromeos%': '<(chromeos)',
     'chromecast%': '<(chromecast)',
     'enable_viewport%': '<(enable_viewport)',
     'enable_hidpi%': '<(enable_hidpi)',
     'enable_topchrome_md%': '<(enable_topchrome_md)',
@@ skipping 5176 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',
   },
 }