[IME] Fix s_suppressNextKeypressEvent logic in WebViewImpl::setComposition().

This implements suzhe@'s suggested fix, as alternative to the attempt in https://codereview.chromium.org/956133002/ .

Also ported the new test over.

BUG=453499

Review URL: https://codereview.chromium.org/970743002

git-svn-id: svn://svn.chromium.org/blink/trunk@191167 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed