Switch rwt to run under python3 for Win, Linux

Heard there are performance improvement in py3. Let's see
if this will make blink_web_tests run faster.

Bug: 1231310
Change-Id: I3d7f9d83d457fef83f7bed28423b84336290fc8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3133901
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Weizhong Xia <[email protected]>
Cr-Commit-Position: refs/heads/main@{#917029}
diff --git a/BUILD.gn b/BUILD.gn
index 3b6472ed..9c0bd12 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1221,7 +1221,9 @@
 
   # https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_tests.md
   script_test("blink_web_tests") {
-    run_under_python2 = true
+    if (is_fuchsia || is_mac) {
+      run_under_python2 = true
+    }
     script = _common_web_test_script
 
     args = _common_web_test_args