RenderTextWin: Break runs at UBLOCK_MISCELLANEOUS_SYMBOLS.

Break runs for symbols like musical note ♫ '\x9835'.
Breaking runs prevents using fallback fonts excessively.
This CL builds on the prior work of http://crrev.com/232076
See before/after pics at http://crbug.com/278913#c21
Add IsUnusualBlockCode helper function; refactor slightly.

BUG=278913
TEST=The musical note symbols, etc. Do not break fonts for subsequent normal text. (Copy and paste "♫" into a bookmark title or similar).
R=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/46873006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233003 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233003