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]

Review URL: https://codereview.chromium.org/1175603002.

Cr-Commit-Position: refs/heads/master@{#334785}
15 files changed