Uses and returns the fractional width in text eliding

ui/gfx/platform_font_mac.mm

Index: ui/gfx/platform_font_mac.mm
diff --git a/ui/gfx/platform_font_mac.mm b/ui/gfx/platform_font_mac.mm
index c75bf66f0a0f0912d2d4e0aadbb342fe2e9b047e..ef3b1d10d65823741c5c57a975e260ad79d42ea0 100644
--- a/ui/gfx/platform_font_mac.mm
+++ b/ui/gfx/platform_font_mac.mm
@@ -12,6 +12,7 @@
 #include "base/strings/utf_string_conversions.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/font.h"
+#include "ui/gfx/font_list.h"
 
 namespace gfx {
 
@@ -72,6 +73,11 @@ int PlatformFontMac::GetStringWidth(const base::string16& text) const {
                                 Font(const_cast<PlatformFontMac*>(this)));
 }
 
+float PlatformFontMac::GetStringWidthF(const base::string16& text) const {
+  return Canvas::GetStringWidthF(
+      text, FontList(Font(const_cast<PlatformFontMac*>(this))));
+}
+
 int PlatformFontMac::GetExpectedTextWidth(int length) const {
   return length * average_width_;
 }