GN: Add rules for webkit_unit_tests (static build)

This adds GN rules for the webkit_unit_tests target in the static build.
The component build links the unit test files into the blink DLL itself
in a somewhat complex way.

This test target doesn't yet link since it depends on symbols provided
by the chromium repo in content/test that haven't been GN-ified, so
it is disabled. This patch does enable blink_heap_unittests and
blink_platform_unittests

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/348763004

git-svn-id: svn://svn.chromium.org/blink/trunk@176619 bbb929c8-8fbe-4397-9dbb-9b2b20218538