WasmGraphBuilder::GrowMemory does not access the module or the instance
any more. This was initially needed to reference the context as a
HeapConstant in the code.
This CL just removes the DCHECKs, which failed with the
--wasm-lazy-compilation flag.

R=ahaas@chromium.org
BUG=chromium:726665

Change-Id: Ieac53fe376256c47e8ef2fafca818a99ff063683
Reviewed-on: https://chromium-review.googlesource.com/516706Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45556}