Add minimum size for using distance fields.

Add minimum size for using distance fields.

Since we're not scaling down as far, we can increase the base
sizes a little.

Also darken debug text colors to get better value comparison 
with black.

BUG=skia:3541
Committed: https://skia.googlesource.com/skia/+/24ba00825092be0d400074e0121ffc7221950dd9

[jvanverth1, 2015-03-16 18:26:59]

jvanverth@google.com changed reviewers:
+ egdaniel@google.com, joshualitt@google.com

[egdaniel, 2015-03-16 18:37:02]

lgtm

[jvanverth1, 2015-03-17 13:23:45]

The CQ bit was checked by jvanverth@google.com

[commit-bot: I haz the power, 2015-03-17 13:23:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1001183003/20001

[jvanverth1, 2015-03-17 13:25:36]

The CQ bit was unchecked by jvanverth@google.com

[jvanverth1, 2015-03-19 18:29:07]

The CQ bit was checked by jvanverth@google.com

[jvanverth1, 2015-03-19 18:29:07]

The patchset sent to the CQ was uploaded after l-g-t-m from egdaniel@google.com
Link to the patchset: https://codereview.chromium.org/1001183003/#ps40001
(title: "Tweak dftext sharpening and smallest size")

[commit-bot: I haz the power, 2015-03-19 18:29:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1001183003/40001

[commit-bot: I haz the power, 2015-03-19 18:34:16]

Committed patchset #3 (id:40001) as
https://skia.googlesource.com/skia/+/24ba00825092be0d400074e0121ffc7221950dd9