Prefer Khronos EGL headers instead Mesa for all OS

This patch change the order that gl target reads Mesa headers, giving
precedence to third_party/khronos over third_party/mesa/src/include.

BUG=266310
TEST=locally on Linux only

Review URL: https://codereview.chromium.org/184433003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255915 0039d316-1c4b-4281-b951-d872f2087c98

ui/gl/gl.gyp

@@ -29,6 +29,7 @@
       ],
       'include_dirs': [
         '<(DEPTH)/third_party/swiftshader/include',
+        '<(DEPTH)/third_party/khronos',
         '<(DEPTH)/third_party/mesa/src/include',
         '<(gl_binding_output_dir)',
       ],