Update comment about is_web_platform_tests_mode_.

This CL updates a comment in in test_runner.h to reflect the fact that:
 - imported/ has been renamed to external/.
 - csswg-test/ is being moved to wpt/css/.

Change-Id: I158c6da888b9af8a123047ffe8369e3c88217792
Reviewed-on: https://chromium-review.googlesource.com/462340
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#460614}

content/shell/test_runner/test_runner.h

@@ -656,7 +656,7 @@ class TestRunner : public WebTestRunner {
   // is ok, because this is taken care of in WebTestDelegate::SetFocus).
   blink::WebView* previously_focused_view_;
 
-  // True if we run a test in LayoutTests/imported/{csswg-test,wpt}/.
+  // True when running a test in LayoutTests/external/wpt/.
   bool is_web_platform_tests_mode_;
 
   // An effective connection type settable by layout tests.