Change key code to "MetaLeft" and "MetaRight" for "Meta" keys

According to the key event spec,
https://www.w3.org/TR/uievents-code/#key-alphanumeric-functional, the
key code for the left and right "Meta" keys should be "MetaLeft" and
"MetaRight" not "OSLeft" and "OSRight". Now, we should update them for
the wpt key tests.
https://codesearch.chromium.org/chromium/src/third_party/blink/
web_tests/external/wpt/webdriver/tests/perform_actions/key_events.py

I created an issue on webdriver github:
w3c/webdriver#1457
Firefox also has a bug of this:
https://bugzilla.mozilla.org/show_bug.cgi?id=1232918

Bug: 1023962
Change-Id: Iff1a678c991d9c6d3733c22a481cbb653b23df0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913103
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Lan Wei <lanwei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717340}

webdriver/tests/perform_actions/support/keys.py

@@ -379,7 +379,7 @@ class Keys(object):
  "value": u"\ue012",
  },
  "META": {
- "code": "OSLeft",
+ "code": "MetaLeft",
  "ctrl": False,
  "key": "Meta",
  "location": 1,
@@ -631,7 +631,7 @@ class Keys(object):
  "value": u"\ue05c",
  },
  "R_META": {
- "code": "OSRight",
+ "code": "MetaRight",
  "ctrl": False,
  "key": "Meta",
  "location": 2,