libchrome: Fix append bug in file_util::ReadFileToString()

This commit reproduces in the form of an epatch an upstream libchrome
fix to prevent ReadFileToString() from appending to the provided output
string.

Bug was observed in update_engine alephversion pref maintenance
Merge pull request #933 from vcaputo/readfiletostring_clearfirst

libchrome: Fix append bug in file_util::ReadFileToString()