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Python always does reference counting and that handles most of the
objects used by GYP. The GC is used to detect and drop circular
references. If GC is turned off then the heap scanning can be skipped
to save some CPU cycles, and the memory leaked due to circular
references isn't noticeable.

This saves about 1 second out of 16 on Linux in a Z620 workstation.

BUG=None

Review URL: https://codereview.chromium.org/495993002

Cr-Commit-Position: refs/heads/master@{#291287}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291287 0039d316-1c4b-4281-b951-d872f2087c98