SerializedScriptValue shouldn't use bloatedCharacters

Rather than using bloatedCharacters, SerializedScriptValue should just ensure
that its m_data String is really 16 bit. That's almost always the case anyway
because SerializedScriptValue doesn't really use strings. It just uses String
as a way to hold a SharedBuffer. For example, unicode is stored in UTF-8 code
units rather than in UTF-16 code units (i.e., two UTF-8 code units per UChar).

This CL removes the last client of bloatedCharacters().

R=eseidel
BUG=254708

Review URL: https://chromiumcodereview.appspot.com/19775008

git-svn-id: svn://svn.chromium.org/blink/trunk@154548 bbb929c8-8fbe-4397-9dbb-9b2b20218538