[TextBlob] Fall back to TightRunBounds when the font bounds are empty

[TextBlob] Fall back to TightRunBounds when the font bounds are empty

Empty font bounds are likely an indication of a font bug.  As a best
effort, we can use TightRunBounds in this easily detectable case.

Since TightRunBounds only supports kDefault_Positioning currently, the
CL also reinstates handling of kFull_Positioning and
kHorizontal_Positioning (removed in http://crrev.com/858153007).

BUG=507022
R=reed@google.com,bungeman@google.com

Committed: https://skia.googlesource.com/skia/+/b0b45d35e4f30c1f42b6a910dc7f90e1a88de70c

[f(malita), 2015-10-09 20:31:17]

The CQ bit was checked by fmalita@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-09 20:31:21]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1399123002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1399123002/1

[f(malita), 2015-10-09 20:33:25]

This is a respin of https://codereview.chromium.org/1284693002/, with
TightRunBounds expanded to handle kFull_Positioning & kHorizontal_Positioning.

[bungeman-skia, 2015-10-09 20:57:30]

lgtm

[commit-bot: I haz the power, 2015-10-09 20:58:45]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-09 20:58:46]

Dry run: This issue passed the CQ dry run.

[f(malita), 2015-10-09 21:45:38]

The CQ bit was checked by fmalita@chromium.org

[commit-bot: I haz the power, 2015-10-09 21:45:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1399123002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1399123002/1

[commit-bot: I haz the power, 2015-10-09 21:46:31]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/b0b45d35e4f30c1f42b6a910dc7f90e1a88de70c