Revert "Maximize the number of parallel processes for processing & uploading perf data"

This reverts commit a92b95bb.

Reason for revert: suspect this causes dashboard internal error. 
BUG:chromium:873102

Original change's description:
> Maximize the number of parallel processes for processing & uploading perf data
> 
> A big portion of perf data uploading latency is due to I/O operation (uploading),
> hence maximize the number of parallel processes to improve the wall-time.
> 
> Bug:857283
> Change-Id: I0c3246849676a56801fe99846ade7265d13d94c0
> 
> TBR=simonhatch@chromium.org
> 
> Change-Id: I0c3246849676a56801fe99846ade7265d13d94c0
> 
> NOTRY=true  # android_arm64_dbg_recipe flake
> 
> Change-Id: I0c3246849676a56801fe99846ade7265d13d94c0
> Reviewed-on: https://chromium-review.googlesource.com/1170083
> Commit-Queue: Ned Nguyen <nednguyen@google.com>
> Reviewed-by: Ned Nguyen <nednguyen@google.com>
> Reviewed-by: Ethan Kuefner <eakuefner@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#581991}

TBR=simonhatch@chromium.org,eakuefner@chromium.org,nednguyen@google.com

Change-Id: I02d1e84a065bd9c315ec2242fc6a8a0bbd6593a1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 857283
Reviewed-on: https://chromium-review.googlesource.com/1169800Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#582113}

tools/perf/process_perf_results.py

@@ -404,10 +404,8 @@ def _handle_perf_results(
           build_properties, output_json_file, service_account_file))
 
     # Kick off the uploads in mutliple processes
-    # Maximimze the parallelization by setting the number of parallel processes
-    # to the number of invocations. This is because a significant part of
-    # uploading are I/Os bounds.
-    pool = mp.Pool(len(invocations))
+    cpus = mp.cpu_count()
+    pool = mp.Pool(cpus)
     try:
       async_result = pool.map_async(
           _upload_individual_benchmark, invocations)