[test-heap] fixed test that was not allowing gc to collect an object

Bug: chromium:845409
Change-Id: I865826c0e8927aea264a4f9177380ec3bb8aee4f
Reviewed-on: https://chromium-review.googlesource.com/1170764Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
Cr-Commit-Position: refs/heads/master@{#55046}

test/mjsunit/regress/regress-crbug-380671.js

@@ -10,4 +10,5 @@ assertEquals(0xc0000000, buffer.byteLength);
 // mock allocator would allow us to allocate more than the physical memory
 // available on 32bit platforms, leaving the internal counters in an invalid
 // state.
+buffer = null;
 gc();