Revert "[PartitionAlloc] Increase the thread cache limit to 1024 bytes."

This reverts commit c6629fb.

Reason for revert: Deprecated in favor of a more holistic solution,
                   let's go back to the baseline to have better
                   comparisons.

Original change's description:
> [PartitionAlloc] Increase the thread cache limit to 1024 bytes.
>
> Renderers make a lot of large-ish allocations, which is not the case
> for the browser process. For instance, on Linux, running Speedometer
> 2.0, the current limit only allows us a 81% hit rate, vs 98% in the
> browser process. Increasing the limit to 1024 bytes halves the thread
> cache hit rate. See details in the linked bug.
>
> Bug: 1169157
> Change-Id: I5437d6a9bfe5a2ab7c24c876f6f92cf79f2109b2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2738979
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Commit-Queue: Benoit L <lizeb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#860531}

Bug: 1169157
Change-Id: I2277241e62eb9fae558f498bb5aaef954524814f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2748302
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#861629}

base/allocator/partition_allocator/thread_cache.h

@@ -228,16 +228,8 @@ class BASE_EXPORT ThreadCache {
   static constexpr float kDefaultMultiplier = 2.;
   static constexpr uint8_t kSmallBucketBaseCount = 64;
 
-  // TODO(crbug.com/1169157): Tune this further.
-  //
-  // From local experiments on Linux with Speedometer 2.0 (see
-  // crbug.com/1169157), renderers make a lot of allocations up to 1024
-  // bytes. The limit was previously set to 512, which is only good to serve
-  // ~81% of allocations in renderers, vs 98% in the browser process. Increasing
-  // this limit to 1024 catches 90% of allocations, that is halves forced misses
-  // vs 512. However, to get to 98%, it is necessary to set the limit at 8192,
-  // which is likely too high across all processes.
-  static constexpr size_t kSizeThreshold = 1024;
+  // TODO(lizeb): Optimize the threshold.
+  static constexpr size_t kSizeThreshold = 512;
 
  private:
   struct Bucket {