GN: Create a :gles2_interface target to resolve some check errors in //cc.

gles2_interface.h defines a pure virtual interface; targets which use it should
depend on it, but not necessarily on an implementation target. Consequently,
a //gpu/command_buffer/client:gles2_interface target was created to reflect the
dependency on this interface without linking a particular implementation.

This resolves an issue with //cc trying to use this header without having a
dependency on a target which includes it.

Review URL: https://codereview.chromium.org/464153002

Cr-Commit-Position: refs/heads/master@{#294956}
diff --git a/gpu/BUILD.gn b/gpu/BUILD.gn
index 531a9da7..c6c8e27 100644
--- a/gpu/BUILD.gn
+++ b/gpu/BUILD.gn
@@ -56,14 +56,16 @@
     "command_buffer/client/gles2_interface_stub.h",
   ]
 
-  configs += [ "//third_party/khronos:khronos_headers" ]
-
   deps = [
     ":gpu",
+    "//gpu/command_buffer/client:gles2_interface",
     "//testing/gmock",
     "//testing/gtest",
     "//ui/gl:gl_unittest_utils",
   ]
+  forward_dependent_configs_from = [
+    "//gpu/command_buffer/client:gles2_interface",
+  ]
 }
 
 test("gl_tests") {