Bug 909887 - Initialize mLastMetrics.mDevPixelsPerCSSPixel to a sane value. r=botond

dom/ipc/TabChild.cpp

@@ -425,6 +425,7 @@ TabChild::Observe(nsISupports *aSubject,
         mLastMetrics.mViewport = CSSRect(CSSPoint(), kDefaultViewportSize);
         mLastMetrics.mCompositionBounds = ScreenIntRect(ScreenIntPoint(), mInnerSize);
         mLastMetrics.mZoom = mLastMetrics.CalculateIntrinsicScale();
+        mLastMetrics.mDevPixelsPerCSSPixel = CSSToLayoutDeviceScale(mWidget->GetDefaultScale());
         // We use ScreenToLayerScale(1) below in order to turn the
         // async zoom amount into the gecko zoom amount.
         mLastMetrics.mCumulativeResolution =