Add a new field "source" in launchData of chrome.app.runtime.onLaunched() to trace launch source.

extensions/common/switches.cc

Index: extensions/common/switches.cc
diff --git a/extensions/common/switches.cc b/extensions/common/switches.cc
index 31f9d28fbe8c33abc6d7be56278ded70a3492f2d..025e237393f3ed5343eac7ab271055242ebf2757 100644
--- a/extensions/common/switches.cc
+++ b/extensions/common/switches.cc
@@ -95,6 +95,9 @@ const char kShowComponentExtensionOptions[] =
 // Adds the given extension ID to all the permission whitelists.
 const char kWhitelistedExtensionID[] = "whitelisted-extension-id";
 
+// Pass launch source to platform apps.
+const char kTraceAppSource[] = "trace-app-source";
+
 }  // namespace switches
 
 }  // namespace extensions