Changes magic sequence to '!#mojo ' and strips 'mojo ' when building url

Prior to this change the magic sequence was '#!mojo:' and the url of the content handler app included mojo:. This works only for mojo: urls, but we want
other schemes. So, the magic sequence is now '#!mojo ' and 'mojo ' is not part of the resulting url.

R=aa@chromium.org

Review URL: https://codereview.chromium.org/821403003

engine/src/flutter/examples/home.sky

@@ -1,4 +1,4 @@
-#!mojo:sky_viewer
+#!mojo mojo:sky_viewer
 <div><!-- remove this element once Document can have multiple element children -->
 <style>
  .h1 { font-size: 2em; margin: 1em; }