WebFrameImpl shouldn't use bloatedCharacters

Instead, it should use appendTextToStringBuilder.

R=eseidel
BUG=254708

Review URL: https://chromiumcodereview.appspot.com/19572010

git-svn-id: svn://svn.chromium.org/blink/trunk@154453 bbb929c8-8fbe-4397-9dbb-9b2b20218538