This reverts commit 99cb4d35.

Reason for revert:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20nosnap%20-%20debug/builds/16445

Original change's description:
> [cleanup] Harden the SubString CSA/Runtime implementations.
> 
> Remove the self-healing for invalid parameters in the
> CodeStubAssembler::SubString helper and the %SubString runtime function,
> which is used as a fallback for the CodeStubAssembler implementation.
> All call sites must do appropriate parameter validation anyways now that
> the self-hosted JavaScript builtins using these helpers are gone, and we
> have proper contracts with the uses.
> 
> Also remove the context parameter from the CodeStubAssembler::SubString
> method, which is unnecessary, since this can no longer throw an
> exception.
> 
> Bug: v8:5269, v8:6936, v8:7109, v8:7137
> Change-Id: I19d93bad5f41faa0561c4561a48f78fcba99a549
> Reviewed-on: https://chromium-review.googlesource.com/795720
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#49702}

TBR=jgruber@chromium.org,bmeurer@chromium.org

Change-Id: I2900b5f087e78f1d321724f03bd063a5ff094183
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:5269, v8:6936, v8:7109, v8:7137
Reviewed-on: https://chromium-review.googlesource.com/796150Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49703}