Fix the build with a GL ES2 configured Qt.

GLES/gl2.h is included through Qt public headers and the copy of
Chromium is used since its include path comes before /usr/include.
The problem is that this header is incompatible for some reasons,
one of them being that it converts all GL function symbols from
gl* to GLES2*.

Qt layer code should always need to go through GL directly,
so make sure that only GYP targets that depend directly on gpu.gyp,
khronos.gyp or webkit_gpu.gyp will have an include path
pointing to those headers.

Replace all_dependent_settings with direct_dependent_settings and
control which target inherits this include_dirs from its dependencies
by using export_dependent_settings.

Change-Id: Id4d98fe22ef8b778b5ba8da300dad28e69507732
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>