https://chromium-review.googlesource.com/971881 triggered jumbo
build failures due to a collision between the FlushICache functions
in module-compiler.cc and the FlushICache enum.  If we move the
enum inside the WasmCode class we can disambiguate references to it.

Change-Id: Icd389ba8abf6afefc4a8aa53887779f4d1357dd2
Reviewed-on: https://chromium-review.googlesource.com/974261
Commit-Queue: Mostyn Bramley-Moore <mostynb@vewd.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52144}