[Ash] ShapedAppWindowTargeter treats empty shapes as empty hit test masks.

Previously, setShape({'rects': []}) or
setShape({'rects': [{left:0, width:0, top:0, height:1}]})
would make the window completely transparent, but still capture input.

This was because ShapedAppWindowTargeter::GetHitTestMask returned
false if the shape was empty. It now returns false only if the shape
is NULL.

BUG=417014

Review URL: https://codereview.chromium.org/602013002

Cr-Commit-Position: refs/heads/master@{#296975}

chrome/browser/ui/views/apps/shaped_app_window_targeter.cc

@@ -18,5 +18,9 @@ ShapedAppWindowTargeter::~ShapedAppWindowTargeter() {
 bool ShapedAppWindowTargeter::GetHitTestMask(aura::Window* window,
                                              gfx::Path* mask) const {
   SkRegion* shape = app_window_->shape();
-  return shape ? shape->getBoundaryPath(mask) : false;
+  if (!shape)
+    return false;
+
+  shape->getBoundaryPath(mask);
+  return true;
 }