AI 144931: Added a (hidden) way to "pre-cache" register maps.

  The 50 methods that appeared on the GC stacks of the most applications
  require 13KB of native heap for their uncompressed register maps, and
  the full set took 5ms to uncompress.  Pre-computation doesn't represent
  a significant improvement in space or time, at the cost of a big pile
  of strings in ZygoteInit.
  I'm leaving the method in ZygoteInit, but it's not called, and the
  static final String[] of method descriptors is empty.  We may want to
  revisit this later.
  BUG=1729570

Automated import of CL 144931