Revert "Limit number of concurrent gsutil cp calls"

This reverts commit d3cfea0c.

Reason for revert: using self.GSUTIL_CP_SEMAPHORE instead of
global GSUTIL_CP_SEMAPHORE

Original change's description:
> Limit number of concurrent gsutil cp calls
>
> gsutil utilizes as many cores as possible, and will create many threads
> too. In combination with gclient doing the same thing (utilize as many
> cores as possible), this results in creating way too many processes and
> some builders are running out of resources (e.g. can't create threads).
>
> This limits number of gsutil cp commands in git_cache to two.
>
> R=​apolito@google.com, dpranke@google.com
>
> Bug: 1255228, 1249003
> Change-Id: Ifcccaacde6c9e1c60c1da0a0bc5a3e512e158dcc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3219900
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

Bug: 1255228, 1249003
Change-Id: I0fde2477ea620c683b49efaa2a70262bd8e615e2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3221900
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

gclient_scm.py

@@ -32,6 +32,12 @@ import shutil
 import subprocess2
 
 
+THIS_FILE_PATH = os.path.abspath(__file__)
+
+GSUTIL_DEFAULT_PATH = os.path.join(
+    os.path.dirname(os.path.abspath(__file__)), 'gsutil.py')
+
+
 class NoUsableRevError(gclient_utils.Error):
   """Raised if requested revision isn't found in checkout."""
 

git_cache.py

@@ -35,12 +35,6 @@ GC_AUTOPACKLIMIT = 50
 
 GIT_CACHE_CORRUPT_MESSAGE = 'WARNING: The Git cache is corrupt.'
 
-# gsutil creates many processes and threads. Creating too many gsutil cp
-# processes may result in running out of resources, and may perform worse due to
-# contextr switching. This limits how many concurrent gsutil cp processes
-# git_cache runs.
-GSUTIL_CP_SEMAPHORE = threading.Semaphore(2)
-
 try:
   # pylint: disable=undefined-variable
   WinErr = WindowsError
@@ -312,7 +306,6 @@ class Mirror(object):
       self.print('Downloading files in %s/* into %s.' %
                  (latest_dir, tempdir))
       with self.print_duration_of('download'):
-        with self.GSUTIL_CP_SEMAPHORE():
         code = gsutil.call('-m', 'cp', '-r', latest_dir + "/*",
                            tempdir)
       if code: