Don't hard-code usage string in mojo_run.

This easily gets out of date - e.g. it contains a '--sky' flag that went
away when the repos split and does not contain most of the actual script
arguments. Instead, describe notable shell arguments in the description
string and allow argparse to generate USAGE.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1343933003 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: fa3d04fd42c795a12aeaf07910b03e5708813e8b

mojo_run

diff --git a/mojo_run b/mojo_run
index 0982ee6..06bc627 100755
--- a/mojo_run
+++ b/mojo_run
@@ -10,28 +10,22 @@
 from devtoolslib import shell_arguments
 from devtoolslib import shell_config
 
-_USAGE = ("mojo_run "
-         "[--args-for=<mojo-app>] "
-         "[--content-handlers=<handlers>] "
-         "[--enable-external-applications] "
-         "[--disable-cache] "
-         "[--enable-multiprocess] "
-         "[--wait-for-debugger] "
-         "[--sky <mojo-app>|<mojo-app>] "
-         """
-
-A <mojo-app> is a Mojo URL or a Mojo URL and arguments within quotes.
-Example: mojo_run "mojo:js_standalone test.js".
-<url-lib-path> is searched for shared libraries named by mojo URLs.
-The value of <handlers> is a comma separated list like:
-text/html,mojo:html_viewer,application/javascript,mojo:js_content_handler
-""")
-
 _DESCRIPTION = """Runner for Mojo applications.
 
 Any arguments not recognized by the script will be passed on as shell arguments.
-"""
 
+Important shell arguments include:
+  "--args-for=<mojo-app-url> <arguments>"
+  "--content-handlers=<handlers>"
+  "--disable-cache"
+  "--enable-multiprocess"
+  "--wait-for-debugger"
+  "<mojo-app-url>"
+  "<mojo-app-url> <arguments>"
+
+The value of <handlers> is a comma separated list like:
+text/html,mojo:html_viewer,application/javascript,mojo:js_content_handler
+"""
 
 # Port on which the mojo:debugger http server will be available on the host
 # machine.
@@ -58,7 +52,9 @@
 def main():
   logging.basicConfig()
 
-  parser = argparse.ArgumentParser(usage=_USAGE, description=_DESCRIPTION)
+  parser = argparse.ArgumentParser(
+      formatter_class=argparse.RawDescriptionHelpFormatter,
+      description=_DESCRIPTION)
   shell_config.add_shell_arguments(parser)
 
   parser.add_argument('--embed', type=str,