Adds support for headless chrome embedder mojo services

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 3cb989d5ae28b7f4c3b76487cb4f909bf0237559..cd9c3434a0442222adf38463a0df6e9daf169e8f 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -5900,11 +5900,14 @@ void RenderFrameImpl::SendUpdateState() {
 
 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
@@ -5914,9 +5917,11 @@ void RenderFrameImpl::MaybeEnableMojoBindings() {
 
   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);
   }
 }