Make DrMemory update process reliable

The DrMemory update process only runs when it thinks that it needs to
but its heuristic is not accurate. A better thing to do is to always
update.

This change also adds setlocal so that environment variables don't
leak outside. This may also help with making this script more robust
in the future, once it has been in place for long enough for all
machines affected by the environment variable leakage to have rebooted.

BUG=595867
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/1807013004 .

Cr-Commit-Position: refs/heads/master@{#381870}

tools/valgrind/chrome_tests.bat

@@ -3,7 +3,8 @@
 :: Use of this source code is governed by a BSD-style license that can be
 :: found in the LICENSE file.
 
-:: TODO(timurrrr): batch files 'export' all the variables to the parent shell
+setlocal
+
 set THISDIR=%~dp0
 set TOOL_NAME="unknown"
 
@@ -32,7 +33,6 @@ echo "Unknown tool: `%TOOL_NAME%`! Only tsan and drmemory are supported right no
 exit /B 1
 
 :SETUP_DRMEMORY
-if NOT "%DRMEMORY_COMMAND%"=="" GOTO :RUN_TESTS
 :: Set up DRMEMORY_COMMAND to invoke Dr. Memory {{{1
 set DRMEMORY_PATH=%THISDIR%..\..\third_party\drmemory
 set DRMEMORY_SFX=%DRMEMORY_PATH%\drmemory-windows-sfx.exe