Add UMA for the editing API document.execCommand.

Add a UMA histogram metrics counting commands when document.execCommand is called.
Each histogram id corresponding to a command name is in EditorCommand.cpp:
https://src.chromium.org/viewvc/blink?revision=165849

BUG=338241

Review URL: https://codereview.chromium.org/238703002

git-svn-id: svn://svn.chromium.org/blink/trunk@172020 bbb929c8-8fbe-4397-9dbb-9b2b20218538