[IME] Supports options page for each IME instead of single options page for each IME extension.

extensions/common/manifest_constants.cc

Index: extensions/common/manifest_constants.cc
diff --git a/extensions/common/manifest_constants.cc b/extensions/common/manifest_constants.cc
index b9db59896e6b0a0a09fe744fa0fa8b6a62fd9447..9e59b441eee7723886b9fb3cd9c203e131f081fd 100644
--- a/extensions/common/manifest_constants.cc
+++ b/extensions/common/manifest_constants.cc
@@ -51,6 +51,7 @@ const char kHideBookmarkButton[] = "hide_bookmark_button";
 const char kHomepageURL[] = "homepage_url";
 const char kIcons[] = "icons";
 const char kId[] = "id";
+const char kImeOptionsPage[] = "options_page";
 const char kImport[] = "import";
 const char kIncognito[] = "incognito";
 const char kIncludeGlobs[] = "include_globs";