linux_aura: Fix abs() warning found by new clang.

BUG=374651

Review URL: https://codereview.chromium.org/292853002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271577 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/ui/libgtk2ui/gtk2_ui.cc

@@ -246,9 +246,12 @@ void PickButtonTintFromColors(const GdkColor& accent_gdk_color,
   // 125] will tint green instead of gray). Slight differences (+/-10 (4%) to
   // all color components) should be interpreted as this color being gray and
   // we should switch into a special grayscale mode.
-  int rb_diff = abs(SkColorGetR(accent_color) - SkColorGetB(accent_color));
-  int rg_diff = abs(SkColorGetR(accent_color) - SkColorGetG(accent_color));
-  int bg_diff = abs(SkColorGetB(accent_color) - SkColorGetG(accent_color));
+  int rb_diff = abs(static_cast<int>(SkColorGetR(accent_color)) -
+                    static_cast<int>(SkColorGetB(accent_color)));
+  int rg_diff = abs(static_cast<int>(SkColorGetR(accent_color)) -
+                    static_cast<int>(SkColorGetG(accent_color)));
+  int bg_diff = abs(static_cast<int>(SkColorGetB(accent_color)) -
+                    static_cast<int>(SkColorGetG(accent_color)));
   if (rb_diff < 10 && rg_diff < 10 && bg_diff < 10) {
     // Our accent is white/gray/black. Only the luminance of the accent color
     // matters.