Fix composition underline offsets for setCompositionFromExistingText

This patch changes calculation of composition underline offsets in InputMethodController::setCompositionFromExistingText() for WebView::SetCompositionFromExistingText() API by using composition start as origin rather than selection start.

In chromium/src/content/browser/renderer_host/ime_adapter_android.cc, underlines are specified as
  std::vector<WebKit::WebCompositionUnderline> underlines;
  underlines.push_back(WebKit::WebCompositionUnderline(0, end - start, SK_ColorBLACK, false));

BUG=275457
TEST=webkit_unit_tests WebViewTest.SetCompositionFromExistingText
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/23282003

git-svn-id: svn://svn.chromium.org/blink/trunk@156374 bbb929c8-8fbe-4397-9dbb-9b2b20218538