(4) Rename files to snake_case: update BUILD.gn, include paths, header guards, and DEPS entries

Mechanically generated by running this command:

tools_webrtc/do-renames.sh update all-renames.txt && git cl format

Then manually updating:

tools_webrtc/sanitizers/tsan_suppressions_webrtc.cc

Bug: webrtc:10159
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I54824cd91dada8fc3ee3d098f971bc319d477833
Reviewed-on: https://webrtc-review.googlesource.com/c/115653
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26226}
diff --git a/rtc_base/sequenced_task_checker_impl.h b/rtc_base/sequenced_task_checker_impl.h
index 293b1ac..a850b94 100644
--- a/rtc_base/sequenced_task_checker_impl.h
+++ b/rtc_base/sequenced_task_checker_impl.h
@@ -11,7 +11,7 @@
 #ifndef RTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_
 #define RTC_BASE_SEQUENCED_TASK_CHECKER_IMPL_H_
 
-#include "rtc_base/criticalsection.h"
+#include "rtc_base/critical_section.h"
 #include "rtc_base/thread_checker.h"
 
 namespace rtc {