Improve ChromeVox continuous read

R=akihiroota@chromium.org, katie@chromium.org

Fixed: 728093
AX-Relnotes: ChromeVox continuous read (Search+r), is smoother in some types of content that contains formatting spans.
Change-Id: I93dfba82614ffdffccefd1d8cb5cc52e484d2f19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288820


Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787104}