Clarify expectation on GlobalLock

Merge GlobalLock and GlobalLockPod, make member private.
annotate creation of all GlobalLocks with ABSL_CONST_INIT

Bug: None
Change-Id: I29abcc86796ec0e45b15df7d26392309d1bf7324
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156303
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29447}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 8f13b0a..0fee5e0 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -1197,6 +1197,7 @@
       "memory:unittests",
       "third_party/base64",
       "third_party/sigslot",
+      "//third_party/abseil-cpp/absl/base:core_headers",
       "//third_party/abseil-cpp/absl/memory",
     ]
   }