Unify native performanceNow implementation across platforms

Summary:
[Changelog][Internal]

Both iOS and Android platforms are at this point using the same native implementation of `performanceNow`, based on `std::chrono` (it used to be different some time ago).

This diff unifies the implementations, so it comes from one place in C++ code for all platforms.

The context is that I am developing event timing instrumentation and need a consistent way to get current timestamp from either JS or native (C++) side. The latter is now possible via calling `JSExecutor::performanceNow()`, which is guaranteed to be the same as called from JS.

Reviewed By: christophpurrer

Differential Revision: D42267898

fbshipit-source-id: dcb592f37d6567340ea59faddbf3b6d2b8507d50