The hardcoded constant caused a problem for the wasm fuzzer because
when the maximum memory was allocated in a test case, clusterfuzz ran
out of memory. with the command line flag we can set a lower limit
for the fuzzer.

The flag has the value of the constant as its default value, so that
for everything but the fuzzers nothing should change.

R=titzer@chromium.org
BUG=chromium:676888

Review-Url: https://codereview.chromium.org/2626313003
Cr-Commit-Position: refs/heads/master@{#42599}