Linux: use generated shim headers for system mesa.

gpu/command_buffer/build_gles2_cmd_buffer.py

Index: gpu/command_buffer/build_gles2_cmd_buffer.py
diff --git a/gpu/command_buffer/build_gles2_cmd_buffer.py b/gpu/command_buffer/build_gles2_cmd_buffer.py
index 99acbc40cefc5ce812c42f17719c5da823373c40..8ce0c39b612447992aceda41b95191d067843f0f 100755
--- a/gpu/command_buffer/build_gles2_cmd_buffer.py
+++ b/gpu/command_buffer/build_gles2_cmd_buffer.py
@@ -2319,6 +2319,16 @@ _CHROMIUM_SPECIFIC_CONSTANTS = {
 
 
 _CHROMIUM_SPECIFIC_EXTENSIONS = {
+  # TODO(phajdan.jr): Remove this after fixing upstream khronos header
+  # for extension below (it misses e.g. #define GL_ANGLE_instanced_arrays 1
+  # after #ifndef check, which confuses generate_bindings.py).
+  'GL_ANGLE_instanced_arrays': {
+    'functions': [
+      'DrawArraysInstancedANGLE',
+      'DrawElementsInstancedANGLE',
+      'VertexAttribDivisorANGLE',
+    ],
+  },
   'GL_EXT_framebuffer_multisample': {
     'constants': {
       'GL_DRAW_FRAMEBUFFER_BINDING': '0x8CA6',