Make skydb load examples/home.sky by default.

I had to also register for the text/plain mime-type
which turned out to be harder than expected.
Mostly due to my confusion with mojo_shell
using last-argument-wins argument parsing.

R=ianh@google.com

Review URL: https://codereview.chromium.org/672363002

engine/src/flutter/examples/home.sky

@@ -1,7 +1,9 @@
 #!mojo mojo:sky
+<sky>
 <style>
  h1 { font-size: 2em; margin: 1em; }
  p { margin: 0.5em 1em; color: #bcd8f5; font-weight: 900; }
 </style>
 <h1>about:blank</h1>
 <p>Welcome to Sky!</p>
+</sky>

engine/src/flutter/tools/debugger/prompt/prompt.cc

@@ -45,13 +45,17 @@ class Prompt : public mojo::ApplicationDelegate {
     app->ConnectToService("mojo:sky_viewer", &tracing_);
     if (app->args().size() > 1)
       url_ = app->args()[1];
+    else {
+      url_ = "https://raw.githubusercontent.com/domokit/mojo/master/sky/"
+          "examples/home.sky";
+    }
   }
 
   virtual bool ConfigureIncomingConnection(
       mojo::ApplicationConnection* connection) override {
     connection->ConnectToService(&debugger_);
-    if (!url_.empty())
-      Reload();
+    std::cout << "Loading " << url_ << std::endl;
+    Reload();
     ScheduleWaitForInput();
     return true;
   }

engine/src/flutter/tools/skydb

@@ -13,11 +13,18 @@ CONFIG_DIRECTORY = 'Debug'
 MOJO_SHELL_PATH = os.path.abspath(os.path.join(__file__, os.pardir, os.pardir,
     os.pardir, BUILD_DIRECTORY, CONFIG_DIRECTORY, 'mojo_shell'))
 
+SUPPORTED_MIME_TYPES = [
+    'text/html',
+    'text/plain',
+]
+
 def main(args):
+    content_handlers = ['%s,%s' % (mime_type, 'mojo://sky_viewer/')
+        for mime_type in SUPPORTED_MIME_TYPES]
     shell_command = [
         MOJO_SHELL_PATH,
         '--v=1',
-        '--content-handlers=text/html,mojo://sky_viewer/',
+        '--content-handlers=%s' % ','.join(content_handlers),
         '--url-mappings=mojo:window_manager=mojo:sky_debugger',
         'mojo:window_manager',
     ]