Revert of Revert of views: Move TOOLKIT_VIEWS definition in //ui/views/ from root.

Revert of Revert of views: Move TOOLKIT_VIEWS definition in //ui/views/ from root. (patchset #1 id:1 of https://codereview.chromium.org/1192583002/)

Reason for revert:
Unreverting because this CL did not end up being the problem.

Original issue's description:
> Revert of views: Move TOOLKIT_VIEWS definition in //ui/views/ from root. (patchset #6 id:100001 of https://codereview.chromium.org/1175603002/)
>
> Reason for revert:
> Speculative revert to fix win unit_test failures. https://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%281%29/builds/39651
>
> Original issue's description:
> > views: Move TOOLKIT_VIEWS definition in //ui/views/ from root.
> >
> > If 'toolkit_views' gyp/gn flag is toggled, then it shouldn't cause targets that
> > do not depend on views to be rebuilt. To make this happen, remove the definition
> > of the TOOLKIT_VIEWS flag from the top-level config into a config in views, so
> > that only targets that explicitly depend on views get the define, and are
> > affected by the switch.
> >
> > BUG=500004
> > R=ben@chromium.org, sky@chromium.org
> >
> > Committed: https://chromium.googlesource.com/chromium/src/+/c13fc9ff0aedca62ebfdfc27d44c6b5ac204c9d8
>
> TBR=ben@chromium.org,sky@chromium.org,sadrul@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=500004
>
> Committed: https://crrev.com/ecbf7e40116d555c11ee958b6339618920d11c2c
> Cr-Commit-Position: refs/heads/master@{#334820}

TBR=ben@chromium.org,sky@chromium.org,sadrul@chromium.org,phoglund@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=500004
Committed: https://crrev.com/27b328bf38b552b1576c0a0368c00a36be5ec781
Cr-Commit-Position: refs/heads/master@{#334923}

[paulmeyer, 2015-06-17 21:19:31]

Created Revert of Revert of views: Move TOOLKIT_VIEWS definition in //ui/views/
from root.

[commit-bot: I haz the power, 2015-06-17 21:22:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1182053007/1

[commit-bot: I haz the power, 2015-06-17 21:26:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-17 21:28:14]

Patchset 1 (id:??) landed as
https://crrev.com/27b328bf38b552b1576c0a0368c00a36be5ec781
Cr-Commit-Position: refs/heads/master@{#334923}