[libFuzzer] Disable len_control by default if LLVMFuzzerCustomMutator is used.

Summary:
Some custom mutators may not peform well when size restriction is
enforced by len_control. Because of that, it's safer to disable len_control
by default in such cases, but still allow users to enable it manually.
Bug example: https://bugs.chromium.org/p/chromium/issues/detail?id=919530.

Tested manually with LPM-based and regular fuzz targets.

Reviewers: kcc, vitalybuka, metzman

Reviewed By: kcc, metzman

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D63334

llvm-svn: 363443