Include "sharedexclusivelock.cc" in Chromium GN build.

Landing https://codereview.webrtc.org/1675923002/ broke some Chromium FYI bots
because the GN build didn't include "sharedexclusivelock.cc" in that scenario.

This CL moves the files from the non-Chromium block into the common sources
list.

BUG=webrtc:5496

Review URL: https://codereview.webrtc.org/1712773003

Cr-Commit-Position: refs/heads/master@{#11699}

webrtc/base/BUILD.gn

@@ -284,6 +284,8 @@ static_library("rtc_base") {
     "sha1.h",
     "sha1digest.cc",
     "sha1digest.h",
+    "sharedexclusivelock.cc",
+    "sharedexclusivelock.h",
     "signalthread.cc",
     "signalthread.h",
     "sigslot.cc",
@@ -394,8 +396,6 @@ static_library("rtc_base") {
       "rollingaccumulator.h",
       "scopedptrcollection.h",
       "sec_buffer.h",
-      "sharedexclusivelock.cc",
-      "sharedexclusivelock.h",
       "sslconfig.h",
       "sslroots.h",
       "testclient.cc",