Bug 1178847 - Move the code from ChromeProcessController::InitializeRoot to APZCCallbackHelper so it can be reused in the child process. r=botond

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -266,6 +266,36 @@ APZCCallbackHelper::GetOrCreateScrollIdentifiers(nsIContent* aContent,
     return false;
 }
 
+void
+APZCCallbackHelper::InitializeRootDisplayport(nsIPresShell* aPresShell)
+{
+  // Create a view-id and set a zero-margin displayport for the root element
+  // of the root document in the chrome process. This ensures that the scroll
+  // frame for this element gets an APZC, which in turn ensures that all content
+  // in the chrome processes is covered by an APZC.
+  // The displayport is zero-margin because this element is generally not
+  // actually scrollable (if it is, APZC will set proper margins when it's
+  // scrolled).
+  if (!aPresShell) {
+    return;
+  }
+
+  MOZ_ASSERT(aPresShell->GetDocument());
+  nsIContent* content = aPresShell->GetDocument()->GetDocumentElement();
+  if (!content) {
+    return;
+  }
+
+  uint32_t presShellId;
+  FrameMetrics::ViewID viewId;
+  if (APZCCallbackHelper::GetOrCreateScrollIdentifiers(content, &presShellId, &viewId)) {
+    // Note that the base rect that goes with these margins is set in
+    // nsRootBoxFrame::BuildDisplayList.
+    nsLayoutUtils::SetDisplayPortMargins(content, aPresShell, ScreenMargin(), 0,
+        nsLayoutUtils::RepaintMode::DoNotRepaint);
+  }
+}
+
 class FlingSnapEvent : public nsRunnable
 {
     typedef mozilla::layers::FrameMetrics::ViewID ViewID;

gfx/layers/apz/util/APZCCallbackHelper.h

@@ -68,6 +68,10 @@ public:
                                              uint32_t* aPresShellIdOut,
                                              FrameMetrics::ViewID* aViewIdOut);
 
+    /* Initialize a zero-margin displayport on the root document element of the
+       given presShell. */
+    static void InitializeRootDisplayport(nsIPresShell* aPresShell);
+
     /* Tell layout to perform scroll snapping for the scrollable frame with the
      * given scroll id. aDestination specifies the expected landing position of
      * a current fling or scrolling animation that should be used to select

gfx/layers/apz/util/ChromeProcessController.cpp

@@ -38,32 +38,7 @@ ChromeProcessController::ChromeProcessController(nsIWidget* aWidget,
 void
 ChromeProcessController::InitializeRoot()
 {
-  // Create a view-id and set a zero-margin displayport for the root element
-  // of the root document in the chrome process. This ensures that the scroll
-  // frame for this element gets an APZC, which in turn ensures that all content
-  // in the chrome processes is covered by an APZC.
-  // The displayport is zero-margin because this element is generally not
-  // actually scrollable (if it is, APZC will set proper margins when it's
-  // scrolled).
-  nsIPresShell* presShell = GetPresShell();
-  if (!presShell) {
-    return;
-  }
-
-  MOZ_ASSERT(presShell->GetDocument());
-  nsIContent* content = presShell->GetDocument()->GetDocumentElement();
-  if (!content) {
-    return;
-  }
-
-  uint32_t presShellId;
-  FrameMetrics::ViewID viewId;
-  if (APZCCallbackHelper::GetOrCreateScrollIdentifiers(content, &presShellId, &viewId)) {
-    // Note that the base rect that goes with these margins is set in
-    // nsRootBoxFrame::BuildDisplayList.
-    nsLayoutUtils::SetDisplayPortMargins(content, presShell, ScreenMargin(), 0,
-        nsLayoutUtils::RepaintMode::DoNotRepaint);
-  }
+  APZCCallbackHelper::InitializeRootDisplayport(GetPresShell());
 }
 
 void