Bug 1509575 - Expose the isFirstPaint flag on LayerManager. r=kats

So that it's easily available during painting.

The flag is set based on nsIPresShell::mIsFirstPaint, but the pres shell
flag is cleared at the beginning of the paint, so we can't query it from
the pres shell during the paint.

Differential Revision: https://phabricator.services.mozilla.com/D16237

--HG--
extra : moz-landing-system : lando

gfx/layers/Layers.h

@@ -610,6 +610,7 @@ class LayerManager : public FrameRecorder {
    * Flag the next paint as the first for a document.
    */
   virtual void SetIsFirstPaint() {}
+  virtual bool GetIsFirstPaint() const { return false; }
 
   /**
    * Set the current focus target to be sent with the next paint.

gfx/layers/client/ClientLayerManager.h

@@ -134,6 +134,9 @@ class ClientLayerManager final : public LayerManager,
   }
 
   virtual void SetIsFirstPaint() override;
+  virtual bool GetIsFirstPaint() const override {
+    return mForwarder->GetIsFirstPaint();
+  }
 
   virtual void SetFocusTarget(const FocusTarget& aFocusTarget) override;
 

gfx/layers/ipc/ShadowLayers.h

@@ -338,6 +338,7 @@ class ShadowLayerForwarder final : public LayersIPCActor,
    * Flag the next paint as the first for a document.
    */
   void SetIsFirstPaint() { mIsFirstPaint = true; }
+  bool GetIsFirstPaint() const { return mIsFirstPaint; }
 
   /**
    * Set the current focus target to be sent with the next paint.

gfx/layers/wr/WebRenderLayerManager.h

@@ -139,6 +139,7 @@ class WebRenderLayerManager final : public LayerManager {
   }
   virtual bool NeedsComposite() const override { return mNeedsComposite; }
   virtual void SetIsFirstPaint() override { mIsFirstPaint = true; }
+  virtual bool GetIsFirstPaint() const override { return mIsFirstPaint; }
   virtual void SetFocusTarget(const FocusTarget& aFocusTarget) override;
 
   virtual already_AddRefed<PersistentBufferProvider>