Make WebViewImpl::setCompositionFromExistingText not to fire events

This patch changes WebViewImpl::setCompositionFromExistingText() not to fire events by setting composition underlines and composition range without changing DOM nodes. Before this patch, this function changes composition by deleting/inserting text and these changes fire event, HTML5 "input" event and Blink internal events, e.g. textDidChangeInTextField.

BUG=263453
TEST=n/a; webkit_unit_tests WebViewTest.SetCompositionFromExistingText* covers this change.
R=aurimas@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/22785003

git-svn-id: svn://svn.chromium.org/blink/trunk@155925 bbb929c8-8fbe-4397-9dbb-9b2b20218538