Pass start/length at InlineTextBox construction time.

This eliminates all but two ways that m_start/m_len can be
changed after construction (markDirty and offsetRun).

Review URL: https://codereview.chromium.org/622253002

git-svn-id: svn://svn.chromium.org/blink/trunk@183349 bbb929c8-8fbe-4397-9dbb-9b2b20218538