Update the first letter when the first line is changed by adding a new text at its start.

CSS first-letter property does not work properly when the first letter is changed
by DOM scripting. This patch allows us to update the first letter and remaining text when the
first letter is changed.

The original patch was reverted due to use-after-frees on ClusterFuzz.
https://codereview.chromium.org/49943002

BUG=92797
TEST=fast/css/first-letter-block-change.html

Review URL: https://codereview.chromium.org/51953006

git-svn-id: svn://svn.chromium.org/blink/trunk@163903 bbb929c8-8fbe-4397-9dbb-9b2b20218538