cc: Invalidate framebuffer contents when clearing

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index a80feac4ecc9f97f3fd5e731d99a43cc7ea98a99..35cca80bc6532384986c8934632cac2d7b6bfdc7 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -204,6 +204,9 @@ bool GLRenderer::Initialize() {
   capabilities_.using_map_image =
       Settings().use_map_image && context_caps.map_image;
 
+  capabilities_.using_discard_framebuffer =
+      context_caps.discard_framebuffer;
+
   is_using_bind_uniform_ = context_caps.bind_uniform_location;
 
   if (!InitializeSharedObjects())
@@ -293,6 +296,14 @@ void GLRenderer::ClearFramebuffer(DrawingFrame* frame) {
     return;
   }
 
+  if (capabilities_.using_discard_framebuffer) {
+    GLenum attachments[] = {
+        GL_COLOR_EXT
+    };
+    context_->discardFramebufferEXT(
+        GL_FRAMEBUFFER, arraysize(attachments), attachments);
+  }
+
   // On DEBUG builds, opaque render passes are cleared to blue to easily see
   // regions that were not drawn on the screen.
   if (frame->current_render_pass->has_transparent_background)