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

third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp

@@ -317,6 +317,7 @@ int HarfBuzzShaper::HarfBuzzRun::characterIndexForXPosition(float targetX)
 float HarfBuzzShaper::HarfBuzzRun::xPositionForOffset(unsigned offset)
 {
     ASSERT(offset < m_numCharacters);
+    ASSERT(hasGlyphToCharacterIndexes());
     unsigned glyphIndex = 0;
     float position = 0;
     if (rtl()) {