Revert 158496 "Update the first letter when the first line is ch..."

Introduced bad cast regression on ClusterFuzz.

> Update the first letter when the first line is changed by adding a new text at its start.
> 
> Currently, 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.
> 
> BUG=92797
> TEST=fast/css/first-letter-block-change.html
> 
> Review URL: https://chromiumcodereview.appspot.com/14113040

TBR=joone.hur@intel.com

Review URL: https://codereview.chromium.org/25713009

git-svn-id: svn://svn.chromium.org/blink/trunk@158774 bbb929c8-8fbe-4397-9dbb-9b2b20218538