Remove setExclusionRectangle() API from InputMethodContext.

At the latest editor's draft of IME API, this method was dropped.
https://dvcs.w3.org/hg/ime-api/diff/10a3d6ec9336/Overview.html

BUG=238585
TEST=don't break any tests

Review URL: https://codereview.chromium.org/28533002

git-svn-id: svn://svn.chromium.org/blink/trunk@159979 bbb929c8-8fbe-4397-9dbb-9b2b20218538