constructTextRun doesn't require a call to bloatedCharacters()

We can construct a TextRun directly from a String without calling
bloatedCharacters now that we support 8 bit TextRuns.

R=eseidel@chromium.org
BUG=254708

Review URL: https://codereview.chromium.org/19296002

git-svn-id: svn://svn.chromium.org/blink/trunk@154271 bbb929c8-8fbe-4397-9dbb-9b2b20218538