implement the new interface.
The upcoming minidump reader will get minidump data from a
FileReaderInterface. For ease of testing, a string-based implementation
is provided. There wasn’t a good reason to have a separate
StringFileReader and StringFileWriter, so I combined them into a single
StringFile.
TEST=util_test StringFile.*
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/936153002