Revert of [heap] Decouple old generation allocation limit from external memory. (patchset #1 id:1 of https://codereview.chromium.org/2329993002/ )

Reason for revert:
Regressions in telemetry benchmarks:
crbug.com/646819.

Original issue's description:
> [heap] Decouple old generation allocation limit from external memory.
>
> We check for external memory limit in Heap::ReportExternalMemoryPressure.
>
> BUG=chromium:616434
>
> Committed: https://crrev.com/672d079ccba686019fa1457c83b42c2e692ef88b
> Cr-Commit-Position: refs/heads/master@{#39374}

TBR=hpayer@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:616434

Review-Url: https://codereview.chromium.org/2339033005
Cr-Commit-Position: refs/heads/master@{#39442}