[Reland] Remove style spans to follow the styles of the block element

Note: The original CL was reverted because of breaking Win8 build:
https://codereview.chromium.org/2109973002/

This CL removes style spans to follow the styles of the block
element(li, pre, td, and h1~6) when the text of the pasted
or merged element becomes a part of the block element.

BUG=226941
TEST=third_party/WebKit/LayoutTests/editing/deleting/backspace-merge-into-block-element.html

Review-Url: https://codereview.chromium.org/2117663002
Cr-Commit-Position: refs/heads/master@{#403428}
4 files changed