Disable FormControlsBrowserTest.DarkModeTextSelection on Mac.

Looks like Mac10.14[1] and Mac10.15[2] builders produce different
test results so I am disabling the test introduced by
https://crrev.com/c/2623652.

[1]: https://ci.chromium.org/p/chromium/builders/ci/Mac10.14%20Tests/b8858077277866870224
[2]: https://ci.chromium.org/p/chromium/builders/ci/Mac10.15%20Tests/b8858076574903066048

TBR=masonfreed@chromium.org,yuzhehan@chromium.org,avi@chromium.org

Bug: 1166988
Change-Id: Ifee799c23cd8134fe9a341d8e6556fe62a321a8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632267Reviewed-by: Ionel Popescu <iopopesc@microsoft.com>
Commit-Queue: Ionel Popescu <iopopesc@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#843954}

content/browser/form_controls_browsertest.cc

@@ -167,7 +167,7 @@ IN_PROC_BROWSER_TEST_F(FormControlsBrowserTest, Radio) {
 
 // TODO(iopopesc): Re-enable test when there is a resolution for
 // android-bfcache-rel builder producing different results.
-#if defined(OS_ANDROID)
+#if defined(OS_ANDROID) || defined(OS_MAC)
 #define MAYBE_DarkModeTextSelection DISABLED_DarkModeTextSelection
 #else
 #define MAYBE_DarkModeTextSelection DarkModeTextSelection