Redo: Adding a mojo interface to the mojo spy

Redo: Adding a mojo interface to the mojo spy

This is the same as
https://codereview.chromium.org/284743002

Which was reverted because a mojo refactoring won the checking race
and I did not notice the revert.

Original message:

Adding more guts to the mojo spy.

This CL fleshes out the basic architecture of the spy. Introducing SpyServerImpl and enough code to operate the "start" and "stop" commands of the front-end.

Still a quite a few loose ends, specially on teardown.

I updated the design doc to reflect this CL:
https://docs.google.com/a/chromium.org/document/d/11FKYXf9mSohlsgl4JmGlyWE1ScX3DKdssdjub63tkwA/edit?usp=sharing

TBR=darin
BUG=360188
TEST=manual via test/spy_repl_test.html

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=281995

[cpu_(ooo_6.6-7.5), 2014-07-09 04:44:33]

The CQ bit was checked by cpu@chromium.org

[commit-bot: I haz the power, 2014-07-09 04:46:18]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/cpu@chromium.org/379753002/20001

[commit-bot: I haz the power, 2014-07-09 07:16:07]

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

[commit-bot: I haz the power, 2014-07-09 09:07:47]

Change committed as 281995