Use the correct omnibox icon for extension URLs

https://crrev.com/2555063003 added an omnibox icon for extension URLs but
was using the wrong asset. This CL updates the icon with a slightly smaller one
suggested by UI team.

Screenshots:

Current: https://drive.google.com/open?id=0B9q2eN9gDoUIQ2JLSzJiY3dEZXM

New: https://drive.google.com/open?id=0B9q2eN9gDoUId081S1EwcDJTWXc
Review-Url: https://codereview.chromium.org/2601093002
Cr-Commit-Position: refs/heads/master@{#440899}

components/toolbar/toolbar_model_impl.cc

@@ -70,7 +70,7 @@ security_state::SecurityLevel ToolbarModelImpl::GetSecurityLevel(
 gfx::VectorIconId ToolbarModelImpl::GetVectorIcon() const {
 #if !defined(OS_ANDROID) && !defined(OS_IOS)
   if (GetURL().SchemeIs("chrome-extension"))
-    return gfx::VectorIconId::EXTENSION;
+    return gfx::VectorIconId::OMNIBOX_EXTENSION_APP;
 
   switch (GetSecurityLevel(false)) {
     case security_state::NONE: