Change the platform app manifest structure.

chrome/common/extensions/extension_manifest_constants.cc

Index: chrome/common/extensions/extension_manifest_constants.cc
diff --git a/chrome/common/extensions/extension_manifest_constants.cc b/chrome/common/extensions/extension_manifest_constants.cc
index 2fda6da66dc3065b87834824580536ded234c597..2ae9de8911a821a9f0ec598f6ab1eb8f4bb6f308 100644
--- a/chrome/common/extensions/extension_manifest_constants.cc
+++ b/chrome/common/extensions/extension_manifest_constants.cc
@@ -85,7 +85,9 @@ const char kPageActionPopup[] = "popup";
 const char kPageActionPopupPath[] = "path";
 const char kPageActions[] = "page_actions";
 const char kPermissions[] = "permissions";
-const char kPlatformApp[] = "platform_app";
+const char kPlatformAppBackground[] = "app.background";
+const char kPlatformAppBackgroundPage[] = "app.background.page";
+const char kPlatformAppBackgroundScripts[] = "app.background.scripts";
 const char kPlugins[] = "plugins";
 const char kPluginsPath[] = "path";
 const char kPluginsPublic[] = "public";
@@ -475,6 +477,12 @@ const char kPlatformAppFlagRequired[] =
     "Loading platform_app extension type is turned off by default. "

[miket_OOO, 2012/06/07 21:34:19]

If we want to be platform_app-clean after this CL, then this error should be
changed, too.

[Mihai Parparita -not on Chrome, 2012/06/07 21:40:01]

I think there's more strings than just these, so I'll do it in a follow-up CL. I
filed http://crbug.com/131688.

     "You can enable this type with the --enable-platform-apps "
     "command-line flag.";
+const char kPlatformAppInvalidBackgroundPage[] =
+    "Platform app background pages are specified via the "
+    "app.background.page key.";

[miket_OOO, 2012/06/07 21:34:19]

Do we want to call these just "apps" rather than "platform apps"? Or is that too
generic?

[Mihai Parparita -not on Chrome, 2012/06/07 21:40:01]

Same as above (and see discussion in my bug comment about naming).

+const char kPlatformAppInvalidBackgroundScripts[] =
+    "Platform app background scripts are specified via the "
+    "app.background.scripts key.";
 const char kPlatformAppNeedsManifestVersion2[] =
     "Platform apps need manifest_version set to >= 2";
 const char kReservedMessageFound[] =