Include all of web_tests/external in wpt_tests_isolate.

This is a leftover review from crrev.com/c/1677161. It ensures we always include the current
manifest file for WPT.

Bug: 937369
Change-Id: I2200cb5644d1518129e577f8612d1e83a4a79c16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1706640
Auto-Submit: Luke Z <lpz@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678373}

third_party/blink/tools/BUILD.gn

@@ -47,12 +47,9 @@ group("wpt_tests_isolate") {
     "//third_party/blink/tools/update_wpt_output.py",
     "//third_party/blink/tools/blinkpy/",
 
-    # The WPT root directory contains both the test runner codebase
-    # and the tests themselves.
-    "//third_party/blink/web_tests/external/wpt/",
-
-    # Include the checked-in WPT manifest.
-    "//third_party/blink/web_tests/external/WPT_BASE_MANIFEST_6.json",
+    # The web_tests/external directory contains all WPT components including
+    # the test runner codebase, manifest file, and the tests themselves.
+    "//third_party/blink/web_tests/external/",
 
     # Include the various Test Expectations files.
     "//third_party/blink/web_tests/ASANExpectations",