Bug 1312606 - Use live pref values when syncing gfxPrefs to the GPU process r=dvander

gfx/thebes/gfxPrefs.cpp

@@ -76,7 +76,7 @@ gfxPrefs::Pref::OnChange()
   if (auto gpm = gfx::GPUProcessManager::Get()) {
     if (gfx::GPUChild* gpu = gpm->GetGPUChild()) {
       GfxPrefValue value;
-      GetCachedValue(&value);
+      GetLiveValue(&value);
       Unused << gpu->SendUpdatePref(gfx::GfxPrefSetting(mIndex, value));
     }
   }

gfx/thebes/gfxPrefs.h

@@ -112,6 +112,9 @@ public:
     // Returns true if the value is default, false if changed.
     virtual bool HasDefaultValue() const = 0;
 
+    // Returns the pref value as a discriminated union.
+    virtual void GetLiveValue(GfxPrefValue* aOutValue) const = 0;
+
     // Returns the pref value as a discriminated union.
     virtual void GetCachedValue(GfxPrefValue* aOutValue) const = 0;
 
@@ -228,6 +231,10 @@ private:
     const char *Name() const override {
       return Prefname();
     }
+    void GetLiveValue(GfxPrefValue* aOutValue) const override {
+      T value = GetLiveValue();
+      CopyPrefValue(&value, aOutValue);
+    }
     // When using the Preferences service, the change callback can be triggered
     // *before* our cached value is updated, so we expose a method to grab the
     // true live value.