Android WebView: build fixes for use_system_skia.

The skia shim headers were not being included at the front of the path
because of a gyp processing issue, causing it to pick up the bundled
headers instead by mistake; fix the gyp file.

Also, some of the chromium code depends on non-public headers; add the
required include path for this to work until we can fix the code.

BUG=263818,274425

Review URL: https://chromiumcodereview.appspot.com/23064007

git-svn-id: http://src.chromium.org/svn/trunk/src/build@218021 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

shim_headers.gypi

@@ -17,12 +17,19 @@
     '<(shim_headers_path)',
   ],
   'all_dependent_settings': {
+    # Repeating this with different numbers of plusses is unfortunately required
+    # to make sure that even if this include is inside nested conditions/etc, it
+    # still gets inserted at the beginning of the include_dirs list. See
+    # http://crbug.com/263818 for details.
     'include_dirs+++': [
       '<(shim_headers_path)',
     ],
     'include_dirs++++': [
       '<(shim_headers_path)',
     ],
+    'include_dirs+++++': [
+      '<(shim_headers_path)',
+    ],
   },
   'actions': [
     {