TextIterator: Set the initial value of m_shadowDepth correctly.

TextIterator is now able to iterate on a document range spanning multiple
tree scopes, but I forgot to initialize the value of m_shadowDepth when
I landed that change.

This may cause the iteration to exit before completion, if the starting
position is in a shadow root and the iterator needs to exit from the shadow
root to reach the ending position. The patch fixes this issue.

Unit tests are added to verify TextIterator's behavior on such cases.

BUG=365246

Review URL: https://codereview.chromium.org/246793008

git-svn-id: svn://svn.chromium.org/blink/trunk@172348 bbb929c8-8fbe-4397-9dbb-9b2b20218538