aura: Make the Linux version of IconLoader not require GTK+.

This change makes it use webkit_glue::ImageDecoder instead
of gdk-pixbuf and hides various GTK+ theme-related code
behind TOOLKIT_USES_GTK ifdefs.  Chrome OS is currently
loading its own icons from resources (see r86936 and
http://crosbug.com/129) instead of using IconLoader and
IconManager, so this only removes NOTIMPLEMENTED()s on
non-Chrome-OS Linux Aura builds (along with simplifying the
GTK+ version of the code).

BUG=99494
TEST=manual: download icons are still visible on a gtk+ build

Review URL: http://codereview.chromium.org/8501030

git-svn-id: http://src.chromium.org/svn/trunk/src/build@109527 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1278,11 +1278,16 @@
           }],
           ['toolkit_uses_gtk!=1', {
             'sources/': [
-              ['exclude', '_(gtk|xdg)(_unittest)?\\.(h|cc)$'],
+              ['exclude', '_gtk(_unittest)?\\.(h|cc)$'],
               ['exclude', '(^|/)gtk/'],
               ['exclude', '(^|/)gtk_[^/]*\\.(h|cc)$'],
             ],
           }],
+          ['OS!="linux"', {
+            'sources/': [
+              ['exclude', '_xdg(_unittest)?\\.(h|cc)$'],
+            ],
+          }],
           ['use_wayland!=1', {
             'sources/': [
               ['exclude', '_(wayland)(_unittest)?\\.(h|cc)$'],