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
> [email protected], [email protected]
>
> Committed: https://chromium.googlesource.com/chromium/src/+/c13fc9ff0aedca62ebfdfc27d44c6b5ac204c9d8

[email protected],[email protected],[email protected]
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=500004

Review URL: https://codereview.chromium.org/1192583002

Cr-Commit-Position: refs/heads/master@{#334820}
15 files changed