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..e7d2e90e4a41a6af9acca7073d1bb0fa7155e4d8 100644
--- a/ui/gfx/platform_font_mac.mm
+++ b/ui/gfx/platform_font_mac.mm
@@ -218,8 +218,14 @@ 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"]]);
+  // -[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.
+  NSAttributedString* as = [[[NSAttributedString alloc]

[tapted, 2017/05/01 06:48:05]

since this is a bit heavyweight, and barely anything calls
GetExpectedTextWidth(..), can we calculate this inside GetExpectedTextWidth?

the data members should have initializers in the .h file (some code above
becomes redundant), then GetExpectedTextWidth can check whether average_width_
has been changed from 0.

Also we like to avoid sending objects to the autorelease pool if it can be
avoided. We can use scoped_nsobject rather than autorelease to do that.

[Elly Fong-Jones, 2017/05/02 14:41:23]

Done.

+      initWithString:@"abcdefghijklmnopqrstuvwxyz"
+          attributes:@{NSFontAttributeName : font}] autorelease];
+  average_width_ = [as size].width / [as length];

[tapted, 2017/05/01 06:48:05]

rounding is occurring here, and GetExpectedTextWidth() takes a "length" argument
that will magnify the rounding. I think we should just make average_width_ a
float, and explicitly do ceil(average_width_ * length) in GetExpectedTextWidth()

[Elly Fong-Jones, 2017/05/02 14:41:23]

Done.

 
   FontRenderParamsQuery query;
   query.families.push_back(font_name_);