Remove "Search Google for this image" context menu item for <webview>.

The functionality was always broken, so remove it until it gets fixed.
We already do not show Searh google for * keyword item anyway.

BUG=407897
Test=In a chrome app, open a <webview> that loads an image.
Right click on that image.
"Search Google for this image" context menu item shouldn't appear.

Review URL: https://codereview.chromium.org/523023003

Cr-Commit-Position: refs/heads/master@{#292749}

chrome/browser/guest_view/web_view/context_menu_content_type_web_view.cc

@@ -23,6 +23,7 @@ bool ContextMenuContentTypeWebView::SupportsGroup(int group) {
     case ITEM_GROUP_PAGE:
     case ITEM_GROUP_FRAME:
     case ITEM_GROUP_LINK:
+    case ITEM_GROUP_SEARCHWEBFORIMAGE:
     case ITEM_GROUP_SEARCH_PROVIDER:
     case ITEM_GROUP_PRINT:
     case ITEM_GROUP_ALL_EXTENSION: