Adds support for headless chrome embedder mojo services

content/renderer/render_frame_impl.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/render_frame_impl.h"
 
 #include <map>
 #include <string>
 #include <utility>
 #include <vector>
@@ skipping 5882 matching lines @@
   DCHECK(SiteIsolationPolicy::UseSubframeNavigationEntries());
   if (current_history_item_.isNull())
     return;
 
   Send(new FrameHostMsg_UpdateState(
       routing_id_, SingleHistoryItemToPageState(current_history_item_)));
 }
 
 void RenderFrameImpl::MaybeEnableMojoBindings() {
   int enabled_bindings = RenderProcess::current()->GetEnabledBindings();
-  // BINDINGS_POLICY_WEB_UI and BINDINGS_POLICY_MOJO are mutually exclusive.
-  // They both provide access to Mojo bindings, but do so in incompatible ways.
-  const int kMojoAndWebUiBindings =
-      BINDINGS_POLICY_WEB_UI | BINDINGS_POLICY_MOJO;
-  DCHECK_NE(enabled_bindings & kMojoAndWebUiBindings, kMojoAndWebUiBindings);
+  // BINDINGS_POLICY_WEB_UI, BINDINGS_POLICY_MOJO and BINDINGS_POLICY_HEADLESS
+  // are mutually exclusive. They provide access to Mojo bindings, but do so in
+  // incompatible ways.
+  const int kAllBindingsTypes =
+      BINDINGS_POLICY_WEB_UI | BINDINGS_POLICY_MOJO | BINDINGS_POLICY_HEADLESS;
+  DCHECK_EQ((enabled_bindings & kAllBindingsTypes) &
+                ((enabled_bindings & kAllBindingsTypes) - 1),

[Charlie Reis, 2016/06/27 17:30:11]

Am I misreading this?  It looks like it's checking x & (x-1) == 0, and I'm not
sure how that equates to checking that enabled_bindings is only one of the
possible values (and not multiple).  (If that's what it's doing, please add a
comment how it works.)

[alex clarke (OOO till 29th), 2016/07/29 09:06:23]

No :)
If only one of those flags is set then enabled_bindings & kAllBindingsTypes must
be a power of two.
x & (x -1) clears the least significant bit, so if x is a power of two then x &
(x -1) will be zero.

e.g.  in binary
let x = 000001000
x - 1 = 000000111
x & (x -1) = 00000000 

let x = 00001010
x - 1 = 00001001
x & (x - 1) = 00001000  

I've added a comment.

PS a great resource for bit-twiddling tricks: 
https://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2

+            0);
 
   // If an MojoBindingsController already exists for this RenderFrameImpl, avoid
   // creating another one. It is not kept as a member, as it deletes itself when
   // the frame is destroyed.
   if (RenderFrameObserverTracker<MojoBindingsController>::Get(this))
     return;
 
   if (IsMainFrame() &&
       enabled_bindings & BINDINGS_POLICY_WEB_UI) {
-    new MojoBindingsController(this, false /* for_layout_tests */);
+    new MojoBindingsController(this, MojoBindingsType::FOR_UI_BINDINGS);
   } else if (enabled_bindings & BINDINGS_POLICY_MOJO) {
-    new MojoBindingsController(this, true /* for_layout_tests */);
+    new MojoBindingsController(this, MojoBindingsType::FOR_LAYOUT_TESTS);
+  } else if (enabled_bindings & BINDINGS_POLICY_HEADLESS) {
+    new MojoBindingsController(this, MojoBindingsType::FOR_HEADLESS);
   }
 }
 
 void RenderFrameImpl::SendFailedProvisionalLoad(
     const blink::WebURLRequest& request,
     const blink::WebURLError& error,
     blink::WebLocalFrame* frame) {
   bool show_repost_interstitial =
       (error.reason == net::ERR_CACHE_MISS &&
        base::EqualsASCII(base::StringPiece16(request.httpMethod()), "POST"));
@@ skipping 376 matching lines @@
   // event target. Potentially a Pepper plugin will receive the event.
   // In order to tell whether a plugin gets the last mouse event and which it
   // is, we set |pepper_last_mouse_event_target_| to null here. If a plugin gets
   // the event, it will notify us via DidReceiveMouseEvent() and set itself as
   // |pepper_last_mouse_event_target_|.
   pepper_last_mouse_event_target_ = nullptr;
 #endif
 }
 
 }  // namespace content