Add a helper to replace FBO attachment's getSerial.

Ideally we could store a render target serial in the render target
itself. For now, this helper function allows us to finally stop
exposing rx::RenderTarget from gl::Texture.

BUG=angle:732

Change-Id: I4ffc5e0af6f0542212ad4e74adced21ebbd89462
Reviewed-on: https://chromium-review.googlesource.com/217330
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libGLESv2/Framebuffer.h b/src/libGLESv2/Framebuffer.h
index 70998ff..cc12d22 100644
--- a/src/libGLESv2/Framebuffer.h
+++ b/src/libGLESv2/Framebuffer.h
@@ -119,6 +119,8 @@
 
 // TODO: place this in FramebufferD3D.h
 RenderTarget *GetAttachmentRenderTarget(gl::FramebufferAttachment *attachment);
+unsigned int GetAttachmentSerial(gl::FramebufferAttachment *attachment);
+
 }
 
 #endif   // LIBGLESV2_FRAMEBUFFER_H_