Include wpt_internal/README as a dep of wpt_tests_isolate,

in order to make the wpt_internal directory exist on the bot so
that the manifest can be written into it.

Bug: 1018829
Change-Id: I1c15caf5abd37c50839587f7713d7636d56a5cd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884812
Commit-Queue: Luke Z <lpz@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710002}

third_party/blink/tools/BUILD.gn

@@ -19,6 +19,11 @@ group("wpt_tests_isolate") {
     # the test runner codebase, manifest file, and the tests themselves.
     "//third_party/blink/web_tests/external/",
 
+    # Include a single file from wpt_internal only to initialize that directory.
+    # We don't need to run these tests but still need to create a MANIFEST here.
+    # See crbug.com/1018829 for more details.
+    "//third_party/blink/web_tests/wpt_internal/README",
+
     # Include the various Test Expectations files.
     "//third_party/blink/web_tests/ASANExpectations",
     "//third_party/blink/web_tests/LeakExpectations",