[Fuchsia] Update a reference to an old target.

The //fuchsia:webrunner_pkg was moved to
//fuchsia/runners:web_runner_pkg but a reference to the old target was
still present in //fuchsia:action. This fixes the issue.

Bug: 922635
Change-Id: I29733695ef4dcbe1e84ae1b3be51754ed69b5297
Reviewed-on: https://chromium-review.googlesource.com/c/1435700
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626175}

fuchsia/BUILD.gn

@@ -336,7 +336,7 @@ action("license") {
     "license_file",
     rebase_path(_license_path, root_build_dir),
     "--gn-target",
-    "//fuchsia:webrunner_pkg",
+    "//fuchsia/runners:web_runner_pkg",
     "--gn-out-dir",
     ".",
   ]