Revert "Deprecated option --shell removed"

This reverts commit f681e530.

Reason for revert: Need to revert:
https://chromium-review.googlesource.com/c/v8/v8/+/723420

Original change's description:
> Deprecated option --shell removed
> 
> Bug: v8:6917
> Change-Id: I4262bc98019a3af0fc2bad7ef12b29ba5d4ea5ea
> Reviewed-on: https://chromium-review.googlesource.com/734241
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Commit-Queue: Michał Majewski <majeski@google.com>
> Cr-Commit-Position: refs/heads/master@{#48855}

TBR=machenbach@chromium.org,majeski@google.com

Change-Id: I8a0065883d41f5011934cdf04263e76801f1910e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6917
Reviewed-on: https://chromium-review.googlesource.com/735150Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48868}

tools/testrunner/base_runner.py

@@ -195,6 +195,8 @@ class BaseTestRunner(object):
     parser.add_option("-m", "--mode",
                       help="The test mode in which to run (uppercase for ninja"
                       " and buildbot builds): %s" % MODES.keys())
+    parser.add_option("--shell", help="DEPRECATED! use --shell-dir",
+                      default="")
     parser.add_option("--shell-dir", help="Directory containing executables",
                       default="")
 
@@ -275,6 +277,13 @@ class BaseTestRunner(object):
   def _set_shell_dir(self, options):
     self.shell_dir = options.shell_dir
     if not self.shell_dir:
+      # TODO(majeski): drop this option
+      if options.shell:
+        print "Warning: --shell is deprecated, use --shell-dir instead."
+        self.shell_dir = os.path.dirname(options.shell)
+      else:
+        # If an output dir with a build was passed, test directly in that
+        # directory.
         self.shell_dir = os.path.join(BASE_DIR, self.outdir)
       if not os.path.exists(self.shell_dir):
           raise Exception('Could not find shell_dir: "%s"' % self.shell_dir)