Reland r288013: First cut at "content handling" support in Mojo.

mojo/shell/switches.cc

Index: mojo/shell/switches.cc
diff --git a/mojo/shell/switches.cc b/mojo/shell/switches.cc
index c597218f412db44cd90e83186ef5afe7b21a89b8..cf3697f645a76e11413b9c6bd21643d785dff18a 100644
--- a/mojo/shell/switches.cc
+++ b/mojo/shell/switches.cc
@@ -10,6 +10,10 @@ namespace switches {
 // |ChildProcess::Type|).
 const char kChildProcessType[] = "child-process-type";
 
+// Comma separated list like:
+// text/html,mojo://mojo_html_viewer,application/bravo,https://abarth.com/bravo
+const char kContentHandlers[] = "content-handlers";
+
 // Force dynamically loaded apps / services to be loaded irrespective of cache
 // instructions.
 const char kDisableCache[] = "disable-cache";