First pass at experimental omnibox API. There are plenty of rough edges and

chrome/common/extensions/extension_constants.cc

Index: chrome/common/extensions/extension_constants.cc
diff --git a/chrome/common/extensions/extension_constants.cc b/chrome/common/extensions/extension_constants.cc
index 174701ca4877712fe2ceacf7aeb78254642e54ed..d0a64e1aa2307fa491fc4a2bbd3b1d3f71c33aca 100644
--- a/chrome/common/extensions/extension_constants.cc
+++ b/chrome/common/extensions/extension_constants.cc
@@ -62,6 +62,7 @@ const wchar_t* kWebContent = L"web_content";
 const wchar_t* kWebContentEnabled = L"web_content.enabled";
 const wchar_t* kWebOrigin = L"web_content.origin";
 const wchar_t* kWebPaths = L"web_content.paths";
+const wchar_t* kOmniboxKeyword = L"omnibox_keyword";
 }  // namespace extension_manifest_keys
 
 namespace extension_manifest_values {
@@ -244,6 +245,11 @@ const char* kCannotAccessPage = "Cannot access contents of url \"*\". "
 const char* kCannotScriptGallery = "The extensions gallery cannot be scripted.";
 const char* kWebContentMustBeEnabled = "The 'web_content.enabled' property "
     "must be set to true in order to use any other web content features.";
+const char* kInvalidOmniboxKeyword =
+    "Invalid value for 'omnibox_keyword'.";
+const char* kOmniboxExperimental =
+    "You must request the 'experimental' permission in order to use the"
+    " omnibox API.";
 }  // namespace extension_manifest_errors
 
 namespace extension_urls {