Return correct length for firstLetterLength.

Currently if the text passed into firstLetterLength has a length of 0 we will
return a length of 1. This is wrong as we should be returning a length of 0.

This CL fixes the code to check or the length and then fixes the caller to
correctly handle the cases where needed the length to be 1 instead of the
correct 0.

Also, LayoutTests were added to show we correctly handle a <br> and <wbr>
as the first character in the div when looking for the first-letter renderer.

BUG=425008

Review URL: https://codereview.chromium.org/684213002

git-svn-id: svn://svn.chromium.org/blink/trunk@184658 bbb929c8-8fbe-4397-9dbb-9b2b20218538