ASSERT that characterIndexForXPosition has a GlyphToCharacter vector

ASSERT that the m_glyphToCharacterIndexes vector is available in the
characterIndexForXPosition method.

BUG=423649
R=jln@chromium.org

Review URL: https://codereview.chromium.org/666613002

git-svn-id: svn://svn.chromium.org/blink/trunk@183930 bbb929c8-8fbe-4397-9dbb-9b2b20218538