Fix use-of-uninitialized-value in TransformationMatrix::blend()

Fix use-of-uninitialized-value in TransformationMatrix::blend()

TransformationMatrix::blend() was attempting to blend between non-invertable
matricies. This resulted in garbage stack variables being used.
This patch ensures that blend() will fall back to a 50% step interpolation
when one of the sides are not invertable.

BUG=389316
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=177453

[alancutter (OOO until 2018), 2014-06-27 09:40:53]

Oops, timloh is not an OWNER here.
+leviw: Are you able to review this fix?

[alancutter (OOO until 2018), 2014-07-02 10:50:33]

leviw: Poke.

[leviw_travelin_and_unemployed, 2014-07-02 17:48:41]

Totally. LGTM.

[alancutter (OOO until 2018), 2014-07-03 00:08:39]

The CQ bit was checked by alancutter@chromium.org

[commit-bot: I haz the power, 2014-07-03 00:08:57]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/alancutter@chromium.org/352793004/1

[commit-bot: I haz the power, 2014-07-03 01:19:02]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_gpu on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/win_gpu/builds/28796)

[commit-bot: I haz the power, 2014-07-03 02:07:20]

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

[commit-bot: I haz the power, 2014-07-03 02:07:21]

Try jobs failed on following builders:
  win_gpu on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/win_gpu/builds/28826)

[alancutter (OOO until 2018), 2014-07-03 02:57:32]

The CQ bit was checked by alancutter@chromium.org

[commit-bot: I haz the power, 2014-07-03 02:57:50]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/alancutter@chromium.org/352793004/1

[commit-bot: I haz the power, 2014-07-03 06:50:26]

Change committed as 177453