Revert of Make full GC reduce memory footprint an explicit event in the idle notification handler. (patchset #2 id:20001 of https://codereview.chromium.org/1072363002/)

Reason for revert:
[Sheriff] breaks nosnap with timeouts:
http://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20nosnap/builds/2513
http://build.chromium.org/p/client.v8/builders/V8%20Win32%20-%20nosnap%20-%20shared/builds/6220

Original issue's description:
> Make full GC reduce memory footprint an explicit event in the idle notification handler.
>
> BUG=
>
> Committed: https://crrev.com/845705aa99b6bfa8d264cfda1c3b5f1229802ab5
> Cr-Commit-Position: refs/heads/master@{#27753}

TBR=ulan@chromium.org,rmcilroy@chromium.org,hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1081443002

Cr-Commit-Position: refs/heads/master@{#27755}