base: Stop overloading ReadFileToString()

With the existing code, base::Bind(&base::ReadFileToString) doesn't compile because
the compiler cannot infer the argument's type (2-argument version or
3-argument version).
base::Bind<bool(*)(const base::FilePath&,
std::string*)>(&base::ReadFileToString) compiles but it doesn't look
desirable.

As our style guide discourages function overloading(*), rename the 3-argument version (added in http://crrev.com/157593005) to stop overloading.

*https://google.github.io/styleguide/cppguide.html#Function_Overloading

BUG=None
TBR=sgurun@chromium.org for android_webview, derat@chromium.org for chromeos/accelerometer, bartfab@chromium.org for components/policy, dsinclair@chromium.org for components/tracing, estade@chromium.org for content/browser/webui, sergeyu@chromium.org for remoting

Review URL: https://codereview.chromium.org/1719983005

Cr-Commit-Position: refs/heads/master@{#377209}
12 files changed