Adjust frame pixel width in phantom window

Since the width is given in pixels which is correct
only on the display with the same DSF, the width needs to
be multiplied by phanton window scale / source scale.

BUG=861967
TEST=Manual. See bug for repro step.

Change-Id: Ib073bb705d8998cf07f41f9d646f0c23828b48de
Reviewed-on: https://chromium-review.googlesource.com/1130595Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573550}

ash/frame/default_frame_header.cc

@@ -125,11 +125,14 @@ void DefaultFrameHeader::DoPaintHeader(gfx::Canvas* canvas) {
   flags.setAntiAlias(true);
   if (width_in_pixels_ > 0) {
     canvas->Save();
-    float scale = canvas->UndoDeviceScaleFactor();
-    gfx::Rect rect = ScaleToEnclosingRect(GetPaintedBounds(), scale, scale);
-
-    rect.set_width(width_in_pixels_);
-    TileRoundRect(canvas, flags, rect, static_cast<int>(corner_radius * scale));
+    float layer_scale =
+        target_widget()->GetNativeWindow()->layer()->device_scale_factor();
+    float canvas_scale = canvas->UndoDeviceScaleFactor();
+    gfx::Rect rect =
+        ScaleToEnclosingRect(GetPaintedBounds(), canvas_scale, canvas_scale);
+    rect.set_width(width_in_pixels_ * canvas_scale / layer_scale);
+    TileRoundRect(canvas, flags, rect,
+                  static_cast<int>(corner_radius * canvas_scale));
     canvas->Restore();
   } else {
     TileRoundRect(canvas, flags, GetPaintedBounds(), corner_radius);