Revert "heap: Enable concurrent sweeping"

This reverts commit 9f60900b.

Reason for revert: Likely breaks webgl2_conformance_tests.

Bug: 985185

Original change's description:
> heap: Enable concurrent sweeping
> 
> Bug: 967258
> Change-Id: I322882558aed37154f2cb2ff655b11c8337b10a0
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1702265
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Anton Bikineev <bikineev@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#678406}

TBR=haraken@chromium.org,mlippautz@chromium.org,bikineev@chromium.org

Change-Id: Iac25b3bd59ac3561b98576a463959f8e1726e7d3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 967258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707350Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678587}

third_party/blink/common/features.cc

@@ -330,7 +330,7 @@ const base::Feature kBlinkHeapConcurrentMarking{
     "BlinkHeapConcurrentMarking", base::FEATURE_DISABLED_BY_DEFAULT};
 // Enables concurrently sweeping Blink's heap.
 const base::Feature kBlinkHeapConcurrentSweeping{
-    "BlinkHeapConcurrentSweeping", base::FEATURE_ENABLED_BY_DEFAULT};
+    "BlinkHeapConcurrentSweeping", base::FEATURE_DISABLED_BY_DEFAULT};
 // Enables incrementally marking Blink's heap.
 const base::Feature kBlinkHeapIncrementalMarking{
     "BlinkHeapIncrementalMarking", base::FEATURE_ENABLED_BY_DEFAULT};