Add InputMethodObserver support into InputMethodBase

This is a preparation CL to fix issue 164964.

Following observer callbacks are defined but not yet
supported by ui::InputMethodBase and its sub classes.
- InputMethodObserver::OnCaretBoundsChanged
- InputMethodObserver::OnInputLocaleChanged

This CL makes these callbacks functional for each
sub class of ui::InputMethodBase.

BUG=164964
TEST=ui_unittests --gtest_filter=InputMethodBaseTest.*

Review URL: https://codereview.chromium.org/48393003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231563 0039d316-1c4b-4281-b951-d872f2087c98
void InputMethodBase::OnInputMethodChanged() const {
  TextInputClient* client = GetTextInputClient();
  if (!IsTextInputTypeNone())
    client->OnInputMethodChanged();
}

void InputMethodBase::OnInputMethodChanged() const {
  TextInputClient* client = GetTextInputClient();
  if (!IsTextInputTypeNone())
    client->OnInputMethodChanged();
}
