Revert "[.bat] Remove "depot_tools" override."

This reverts commit 2f5f0b7a99.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [.bat] Remove "depot_tools" override.
> 
> Currently, all ".bat" entry points use "~dp0python" to ensure that the
> Python that is used to execute the tool is the one in depot_tools. This
> prevents any sort of system override.
> 
> Remove this override so that PATH solely determines which Python is
> used. To accommodate users who invoked these tools without Python on the
> PATH, we still still add "depot_tools" as a catch-all PATH suffix.
> 
> Some tools were also not using DOS-style line endings. This CL fixes
> this.
> 
> BUG=chromium:714293, chromium:724902
> TEST=None
> 
> Change-Id: I06e9583a668c767196a2a335547aded868f2a2b5
> Reviewed-on: https://chromium-review.googlesource.com/517236
> Commit-Queue: Daniel Jacques <dnj@chromium.org>
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
> 

TBR=maruel@chromium.org,iannucci@chromium.org,vadimsh@chromium.org,dnj@chromium.org
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
BUG=chromium:714293, chromium:724902

Change-Id: I822abdd4e02abd32d2f4789fb16d5a7f78fdd02d
Reviewed-on: https://chromium-review.googlesource.com/521867
Reviewed-by: Daniel Jacques <dnj@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>

gclient.bat

@@ -7,9 +7,5 @@ setlocal
 :: Synchronize the root directory before deferring control back to gclient.py.
 call "%~dp0update_depot_tools.bat" %*
 
-:: Ensure that "depot_tools" is somewhere in PATH so this tool can be used
-:: standalone, but allow other PATH manipulations to take priority.
-set PATH=%PATH%;%~dp0
-
 :: Defer control.
-"python" "%~dp0gclient.py" %*
+"%~dp0python" "%~dp0gclient.py" %*