Split SkFontHost_win_dw.

Split SkFontHost_win_dw.

Split SkFontHost_win_dw into FontMgr, Typeface, and ScalerContext.
This makes working on these files easier, and moves away from the legacy
FontHost naming.

Committed: https://skia.googlesource.com/skia/+/51daa25a2b16bca578e78b7ea1e5815b9abb8b0b

[bungeman-skia, 2014-06-05 18:47:56]

As much as possible, this is a simple code re-organization with as few code
changes as possible. 

Some follow ups are:
Rename SkFontHost_win_dw.cpp to SkScalerContext_win_dw.cpp (not doing that here
so that git history is sane).

Rename SkScalerContext_DW to SkScalerContext_DirectWrite.

Rename DWriteFontTypeface to SkTypeface_DirectWrite.

More generally, move creating a typeface from a stream out of the font manager
to a factory on the typeface.

The motivation for this is that while working on the Android and FontConfig
based ports the current organization makes it difficult to see which direction
to move in. Since this port was designed around moving in this direction it
seemed best to convert it first to clarify.

[reed1, 2014-06-05 19:41:42]

lgtm

[bungeman-skia, 2014-06-05 20:14:48]

The CQ bit was checked by bungeman@google.com

[commit-bot: I haz the power, 2014-06-05 20:16:07]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/bungeman@google.com/314193002/20001

[commit-bot: I haz the power, 2014-06-05 20:38:50]

Change committed as 51daa25a2b16bca578e78b7ea1e5815b9abb8b0b