GN: Only define UNIT_TEST in direct gtest deps
GN: Only define UNIT_TEST in direct gtest deps

This matches the gyp build and is important for parts of the code that
guard their API with UNIT_TEST. For instance,
chrome/browser/ui/views/omnibox/omnibox_view_views.h has a GetRenderText
function guarded by #if UNIT_TEST that shadows a protected function
GetRenderText from its parent class views::Textfield, but with different
const-ness. If c/b/u/v/o/omnibox_view_views.cc is compiled with UNIT_TEST
set, it attempts to use the guarded member function instead of the base
class which fails to compile.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/464503003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@288988 0039d316-1c4b-4281-b951-d872f2087c98

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