[Fuchsia] Add top-level webrunner directory and webrunner skeleton.

WebRunner will be used on Fuchsia to run web applications.

Bug: 822474
Change-Id: I56b2fd4bf653bb2c77a013b3134f6dba8065289a
Reviewed-on: https://chromium-review.googlesource.com/965255
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Ben Goodger <ben@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547039}

diff --git a/BUILD.gn b/BUILD.gn
index fccc380..5a3538c 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -742,6 +742,7 @@
       "//headless",
       "//headless:headless_shell",
       "//headless:headless_tests",
+      "//webrunner",
     ]
   }
 }