rename getInputContext to .inputMethodContext.

This corresponds to the latest editor's draft at
https://dvcs.w3.org/hg/ime-api/raw-file/97366a9efb85/Overview.html

BUG=268259

Review URL: https://chromiumcodereview.appspot.com/21636005

git-svn-id: svn://svn.chromium.org/blink/trunk@155568 bbb929c8-8fbe-4397-9dbb-9b2b20218538