Several code patching paths actually flushed the icache twice.
This CL removes all icache flushing on wasm updates from
assembler.cc and pushes it into the platform specific
unchecked_update_wasm_size and
unchecked_update_wasm_memory_reference methods. They were already
receiving the ICacheFlushMode, and on some platforms would already
perform the flush.

R=ahaas@chromium.org

Change-Id: I388701f13d733cb5387d5bed4dbed33879179a91
Reviewed-on: https://chromium-review.googlesource.com/450246
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43635}