Get rid of unused API function WebViewClient:: didExecuteCommand()

This patch gets rid of unused API function WebViewClient::didExecuteCommand(),
which is used for counting part of |document.execCommand|. We provide another
counter for |document.execCommand| which counts all commands and partial
counter is no longer used.

Chromium changes will be committed once this patch is rolled into.

BUG=376154
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/292433011

git-svn-id: svn://svn.chromium.org/blink/trunk@174631 bbb929c8-8fbe-4397-9dbb-9b2b20218538