Add the local blink_perf benchmark that supports running any subdir of WebKit/PerformanceTest

This tries to mimic the current commandline opion of
./third_party/WebKit/Tools/Scripts/run-perf-tests. i.e: user now can run
test case in WebKit/PerformanceTests/.. directory using the command:

./tools/perf/run_benchmark blink_perf --test-path=<path to test cases>

For examples, after this CL, users can invoke:
* './tools/perf/run_benchmark blink_perf --test-path=third_party/WebKit/PerformanceTests/TestData/color-changes-measure-frame-time.html'
command to run only color-changes-measure-frame-time.html test.

* './tools/perf/run_benchmark blink_perf --test-path=TestData/'
command to run all the tests in WebKit/PerformanceTests/TestData/ directory.

This is a lot more convenient for users who are wanting to add new test cases in
directories that aren't already covered by existing blink_perf.* benchmarks.

This benchmark is added to tools/perf/contrib/ directory because it isn't meant
to be scheduled on the perf waterfall.

BUG=720772

Review-Url: https://codereview.chromium.org/2874653002
Cr-Commit-Position: refs/heads/master@{#471028}

tools/perf/contrib/blink_perf_cmdline/blink_perf_cmdline.py

+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+import os
+
+from benchmarks import blink_perf
+
+
+# pylint: disable=protected-access
+class BlinkPerfAll(blink_perf._BlinkPerfBenchmark):
+
+  @classmethod
+  def Name(cls):
+    return 'blink_perf'
+
+  @classmethod
+  def AddBenchmarkCommandLineArgs(cls, parser):
+    parser.add_option('--test-path', type='string',
+                      default=blink_perf.BLINK_PERF_BASE_DIR,
+                      help=('Path to blink perf tests. Could be an absolute '
+                            'path, a relative path with respect to your '
+                            'current directory, or a relative path with '
+                            'respect to third_party/WebKit/PerformanceTest/)'))
+
+  def CreateStorySet(self, options):
+    if os.path.exists(options.test_path):
+      path = os.path.abspath(options.test_path)
+    else:
+      path = os.path.join(blink_perf.BLINK_PERF_BASE_DIR, options.test_path)
+    print
+    print 'Running all tests in %s' % path
+    return blink_perf.CreateStorySetFromPath(path, blink_perf.SKIPPED_FILE)