Fix integer overflow.

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
index cfc4f0d6f8c37cf2da4e63a17ef5495930eb79f1..c33ace6a3c165c71b682f615d1753b2eafe90fa9 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
@@ -10,6 +10,7 @@
 #include <X11/Xregion.h>
 #include <X11/Xutil.h>
 
+#include <algorithm>
 #include <utility>
 
 #include "base/command_line.h"
@@ -1092,7 +1093,15 @@ void DesktopWindowTreeHostX11::SetOpacity(float opacity) {
   // X server opacity is in terms of 32 bit unsigned int space, and counts from
   // the opposite direction.
   // XChangeProperty() expects "cardinality" to be long.
-  unsigned long cardinality = static_cast<int>(opacity * 255) * 0x1010101;

[sky, 2017/01/03 17:54:27]

Can you outline where this goes wrong? Before my change the code was this:

void DesktopWindowTreeHostX11::SetOpacity(unsigned char opacity) {
  unsigned long cardinality = opacity * 0x1010101;

But as long as opacity (in the new code) is 0-1, shouldn't this be the same?

+
+  // Clamp opacity to [0.0 .. 1.0] range.
+  float clamped_opacity = std::max(0.0f, std::min(1.0f, opacity));

[sky, 2017/01/03 17:54:27]

We don't do this anywhere else. It's assumed the value is 0-1. A DCHECK here is
fine (or better yet in Widget::SetOpacity() which is generally the entry point
for this.

+  // Scale opacity to [0 .. 255] range.
+  unsigned long opacity_8bit =
+      static_cast<unsigned long>(clamped_opacity * 255.0f);
+  // Use opacity value for all channels.
+  const unsigned long channel_multiplier = 0x1010101;
+  unsigned long cardinality = opacity_8bit * channel_multiplier;
 
   if (cardinality == 0xffffffff) {
     XDeleteProperty(xdisplay_, xwindow_,