Make ReadEntireFileToString return a std::string. Note: this makes the method unavailable for export from angle_util, which is probably why it was designed the way it was in the first place. However, we import the test utils source file as a static lib into each test executable and test shared module, so it works. Bug: angleproject:7404 Change-Id: Ia957268882c2b8529643660d7d4f34d142c0dc43 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3708602 Commit-Queue: Roman Lavrov <romanl@google.com> Reviewed-by: Roman Lavrov <romanl@google.com>