Fix ui/gl and gpu target building with GN on Windows.

ui/gl/BUILD.gn

Index: ui/gl/BUILD.gn
diff --git a/ui/gl/BUILD.gn b/ui/gl/BUILD.gn
index 55218bcf6a6cab44429384158635d2c2ce77b3c6..2af7aa95ddd34a060389c94fdf112a9aab28c721 100644
--- a/ui/gl/BUILD.gn
+++ b/ui/gl/BUILD.gn
@@ -20,6 +20,12 @@ config("gl_config") {
   include_dirs = [ gl_binding_output_dir ]
 }
 
+# Dependents of this component must separatedly add
+# "//third_party/mesa/src/include" to include_dirs. Although this can be done
+# through all_dependent_configs, that does not ensure that this directory will
+# be before Windows SDK include directory. But we must guarantee that, in other
+# case we will pick gl.h header from SDK rather from mesa,
+# and compilation will fail.

[Slava Chigrin, 2014/12/16 17:53:49]

Unfortunatelly, I can not find any more elegant solution. Windows SDK includes
added to most targets in src/build/config/BUILDCONFIG.gn, and they appear in
include list before include dirs for any other configs.

If there is some GN feature, that allows include dir re-order by configs, I'll
be happy to rewrite this patch.

[no sievers, 2014/12/16 19:38:46]

hmm yea gyp has something to inject dependencies for dependents. i'd think GN
would have something too.

[Slava Chigrin, 2014/12/16 22:32:38]

Sorry, let me explain problem, arising here, in more details. Root problem is
that these files pick gl.h header from system SDK, rather then from mesa, and
fails to compile with it. If we pick gl.h from mesa, compilation will succeed.
On Windows We add path to Windows SDK to (almost?) all targets. It is always
gets added in the beginning of compiler command line. If we would want just add
mesa/src/include to the command line, then dependencies (or dependencies for
dependencies) perfectly solve problem. But unfortunately, we need not only add
new path, but add it BEFORE windows SDK path. Seems, in GN this task can be done
only by manually adding it to include_dirs of appropriate targets, since dirs
from dependent configs gets added only AFTER SDK path.

It is certainly possible that I took wrong way somewhere, just tell  me, is it
supposed that these files be able to be build using system gl.h? If no, then is
it possible to re-order config paths using GN configs?

 component("gl") {
   output_name = "gl_wrapper"  # Avoid colliding with OS X"s libGL.dylib.
 
@@ -318,6 +324,8 @@ source_set("gl_unittest_utils") {
     "$gl_binding_output_dir/gl_mock_autogen_gl.h",
   ]
 
+  # See commend before :gl target.
+  include_dirs = [ "//third_party/mesa/src/include" ]

[no sievers, 2014/12/16 19:38:46]

why do we need this? we usually prefer khronos headers I think. and why was the
problem windows-specific?

[Slava Chigrin, 2014/12/16 22:32:38]

OK, will change include path, if khronos have gl.h, that will help. But in any
case, include search order problems remains. I guess, problem is not met on
Linux, since there we just add --sysroot command line option, if required,
instead of adding path to /usr/include to beginning of compiler command line.
Thus, all provided include dirs (mesa, or khronos, as you said) get searched
before compiler goes to examine system SDK dirs. On windows at present it checks
SDK dirs first.

   configs += [ "//third_party/khronos:khronos_headers" ]
   public_configs = [ ":gl_unittest_utils_config" ]