adding baseline options for super/sub scripting

ui/gfx/text_constants.h

Index: ui/gfx/text_constants.h
diff --git a/ui/gfx/text_constants.h b/ui/gfx/text_constants.h
index 82f2a3b7061386b425c825480db256fe8c9ca446..cbd582119cf1403fb8d07024d27c6063083ae4d5 100644
--- a/ui/gfx/text_constants.h
+++ b/ui/gfx/text_constants.h
@@ -32,6 +32,16 @@ enum DirectionalityMode {
 };
 
 // Text styles and adornments.
+// Figure of font metrics:

[msw, 2015/02/17 23:32:06]

nit: vertically center "height", "ascent", and "cap height".

[dschuyler, 2015/02/18 01:37:27]

Done.

+//   +--------+--------+------------------------+-------------+
+//   |        |        | internal leading       | SUPERSCRIPT |
+//   |        | ascent +------------+-----------|             |
+//   |        |        |            | SUPERIOR  |-------------+
+//   |        |        |            |-----------|
+//   | height |        | cap height | INFERIOR  |-------------+
+//   |        |--------+------------+-----------| SUBSCRIPT   |
+//   |        | descent                         |             |
+//   +--------+---------------------------------+-------------+
 // TODO(msw): Merge with gfx::Font::FontStyle.
 enum TextStyle {
   BOLD = 0,
@@ -39,6 +49,10 @@ enum TextStyle {
   STRIKE,
   DIAGONAL_STRIKE,
   UNDERLINE,
+  SUPERSCRIPT,     // e.g. a mathematical exponent would be superscript

[msw, 2015/02/17 23:32:06]

I don't think you'll want to merge these directly with TextStyle. Instead add
something like a BreakList<ScriptBaseline> RenderText::baseline_types_; or
similar for a new enum of these script baseline types, including a NORMAL type.

Otherwise, using a bool value for each style would allow users to set
overlapping ranges with different script baseline styles. I imagine we don't
(yet) want to handle combined/nested script baseline types (eg. a range marked
true for both subscript and superscript, etc.).

[dschuyler, 2015/02/18 01:37:27]

I'm a little less confident for the number of places I ended up adding baseline
changes.  Something I still haven't worked out the implications of is this, "Use
an empty color BreakList to avoid breaking runs at color boundaries."  I guess
I'm wondering if I should do something similar with the baseline styles or not.

Done.

[msw, 2015/02/18 17:07:23]

What you have looks pretty good. That comment is saying that we shouldn't break
runs for text color ranges, so it doesn't use the color breaklist. Since we
*should* break runs for baseline changes (because it uses a different font
size), we'll use the actual baseline style breaklist there (as you currently
do).

+  SUPERIOR,        // e.g. 8th, the "th" would be superior script
+  INFERIOR,        // e.g. 1/2, the "2" would be inferior ("1" is superior)
+  SUBSCRIPT,       // e.g. H2O, the "2" would be subscript
   NUM_TEXT_STYLES,
 };