Store DM failures in an array, not a single string.

Store DM failures in an array, not a single string.

This should fix the problem where a repeatedly failing test causes this assertion:
  ../../src/core/SkString.cpp:572: failed assertion "length >= 0 && length < SkToInt(kBufferSize)"

Example output:

0 GMs x 7 configs, 2 tests, 0 pictures
0 tasks left, 100 failed
Failures:
  test RTree: ../../tests/RTreeTest.cpp:77	0 != rtree.getCount()
  test RTree: ../../tests/RTreeTest.cpp:77	0 != rtree.getCount()
  test RTree: ../../tests/RTreeTest.cpp:77	0 != rtree.getCount()
  test RTree: ../../tests/RTreeTest.cpp:77	0 != rtree.getCount()
  < 95 lines elided >
  test RTree: ../../tests/RTreeTest.cpp:77	0 != rtree.getCount()
100 failures.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/008f1ea5812f6ebe42623c99b5e77dca423a4f1c

[mtklein, 2014-11-19 21:28:34]

mtklein@google.com changed reviewers:
+ bsalomon@google.com, mtklein@google.com

[mtklein, 2014-11-19 21:28:38]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-11-19 21:28:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/741833002/1

[commit-bot: I haz the power, 2014-11-19 21:28:55]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2014-11-20 03:28 UTC

[bsalomon, 2014-11-19 21:30:09]

lgtm

[commit-bot: I haz the power, 2014-11-19 21:36:23]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/008f1ea5812f6ebe42623c99b5e77dca423a4f1c