Instantiate PolicyValueStoreTest/ValueStoreTest.

This wasn't instantiated because the source is in
value_store_unittest.cc, which is a bunch of TEST_P()s that are
instantiated in several other files in different test suites.  However
the file was in .gn as part of just the extensions tests, instead of
being part of test_support, so it wasn't linked into unit_tests, so the
attempted instantiation for PolicyValueStoreTest didn't work.

Bug: none
Change-Id: I5089eeb39a92b1f5e72c5a27c7ad3a2b283bf60b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2139080
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757173}

extensions/browser/BUILD.gn

@@ -549,6 +549,8 @@ jumbo_source_set("test_support") {
     "value_store/test_value_store_factory.h",
     "value_store/testing_value_store.cc",
     "value_store/testing_value_store.h",
+    "value_store/value_store_unittest.cc",
+    "value_store/value_store_unittest.h",
   ]
 
   deps = [
@@ -664,8 +666,6 @@ source_set("unit_tests") {
     "value_store/testing_value_store_unittest.cc",
     "value_store/value_store_change_unittest.cc",
     "value_store/value_store_frontend_unittest.cc",
-    "value_store/value_store_unittest.cc",
-    "value_store/value_store_unittest.h",
     "verified_contents_unittest.cc",
     "warning_service_unittest.cc",
   ]