Use ScaleToEnclosingRect when converting the viewport to window.

This guarantees that the viewport is always contained in the window.

BUG=none

Review-Url: https://codereview.chromium.org/2258493002
Cr-Commit-Position: refs/heads/master@{#414452}

content/renderer/render_widget.cc

@@ -1516,15 +1516,10 @@ void RenderWidget::UpdateCompositionInfo(bool immediate_request) {
 
 void RenderWidget::convertViewportToWindow(blink::WebRect* rect) {
   if (IsUseZoomForDSFEnabled()) {
-    float reverse = 1 / GetOriginalDeviceScaleFactor();
     // TODO(oshima): We may need to allow pixel precision here as the the
     // anchor element can be placed at half pixel.
-    gfx::Rect window_rect =
-        gfx::ScaleToEnclosedRect(gfx::Rect(*rect), reverse);
-    rect->x = window_rect.x();
-    rect->y = window_rect.y();
-    rect->width = window_rect.width();
-    rect->height = window_rect.height();
+    *rect =
+        gfx::ScaleToEnclosingRect(*rect, 1.f / GetOriginalDeviceScaleFactor());
   }
 }