Rename |imp| => |impl| in wtf (one use)

This is the last inconsistent use of |imp| instead of |impl| in Chromium C++.
(There are a few Objective C++ uses that I'm not touching.)

Followup to
Rename |imp| => |impl| in bindings generation
https://codereview.chromium.org/202203009/

TBR=abarth

Review URL: https://codereview.chromium.org/203073002

git-svn-id: svn://svn.chromium.org/blink/trunk@169415 bbb929c8-8fbe-4397-9dbb-9b2b20218538