PlatformFontMac: better guess for average width

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 53781f8e2309e74b7c72fb319b85c430132c83aa..ec234d513916febb8624c96613e3a65f4577bbed 100644
--- a/ui/gfx/platform_font_mac.mm
+++ b/ui/gfx/platform_font_mac.mm
@@ -133,7 +133,18 @@ int GetFontStyleFromNSFont(NSFont* font) {
 }
 
 int PlatformFontMac::GetExpectedTextWidth(int length) {
-  return length * average_width_;
+  if (!average_width_) {

[tapted, 2017/05/03 00:43:35]

I think we need a `&& native_font_` in this condition -- there are weird cases
where it's null for invalid font names and I'm not sure whether
NSAttributedString will behave in that case 

[Elly Fong-Jones, 2017/05/03 18:06:07]

Done.

+    // -[NSFont boundingRectForGlyph:] seems to always return the largest
+    // bounding rect that could be needed, which produces very wide expected
+    // widths for strings. Instead, compute the actual width of a string
+    // containing all the lowercase characters to find a reasonable guess at the
+    // average.
+    base::scoped_nsobject<NSAttributedString> as([[NSAttributedString alloc]
+        initWithString:@"abcdefghijklmnopqrstuvwxyz"
+            attributes:@{NSFontAttributeName : native_font_.get()}]);
+    average_width_ = [as size].width / [as length];

[tapted, 2017/05/03 00:43:35]

nit: DCHECK(!!average_width_) ?

[Elly Fong-Jones, 2017/05/03 18:06:07]

Done.

+  }
+  return ceil(length * average_width_);
 }
 
 int PlatformFontMac::GetStyle() const {
@@ -202,7 +213,6 @@ int GetFontStyleFromNSFont(NSFont* font) {
     height_ = 0;
     ascent_ = 0;
     cap_height_ = 0;
-    average_width_ = 0;
     return;
   }
 
@@ -218,9 +228,6 @@ int GetFontStyleFromNSFont(NSFont* font) {
   // to ensure GetBaseline() + descender fits within GetHeight() during layout.
   height_ = ceil(ascent_ + std::abs([font descender]) + [font leading]);
 
-  average_width_ =
-      NSWidth([font boundingRectForGlyph:[font glyphWithName:@"x"]]);
-
   FontRenderParamsQuery query;
   query.families.push_back(font_name_);
   query.pixel_size = font_size_;