Support paste with style from Spanned

Only coerce text to HTML when needed, since replace getHtmlText with coerceToHtmlText will break two bookmark tests.
1. if text is HTML.
2. if text is plain text and has style (Spanned).

BUG=709549

Review-Url: https://codereview.chromium.org/2812933003
Cr-Commit-Position: refs/heads/master@{#464584}
2 files changed