Fix converted result caching in FileReaderLoader::stringResult()

stringResult() returns cached result when m_rawData is not
modified since the last stringResult() call. But the variable
m_isRawDataConverted which tells if m_rawData is modified and
not yet converted is not correctly set when conversion is done.
This CL fixes it.

Review URL: https://chromiumcodereview.appspot.com/23440029

git-svn-id: svn://svn.chromium.org/blink/trunk@158092 bbb929c8-8fbe-4397-9dbb-9b2b20218538