This CL cleans up a few things:
- It removes two dead declarations: WasmMemoryObject::Grow and
  wasm::GrowInstanceMemory.
- It removes the unneeded wasm::GetInstanceMemory function (use
  instance->memory_buffer() directly).
- It moves wasm::GetInstanceMemorySize to
  WasmInstanceObject::GetMemorySize.
- It moves wasm::GrowInstanceMemory to WasmInstanceObject::GrowMemory.
- It moves wasm::GrowWebAssemblyMemory to WasmMemoryObject::Grow.

R=ahaas@chromium.org
CC=gdeepti@chromium.org

Change-Id: I19781ca9784f1a8e7b60955bef82e341c4f75550
Reviewed-on: https://chromium-review.googlesource.com/463167
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44293}