(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/pc/simulcast_description.h b/pc/simulcast_description.h
index 0781a6a..1337a9c 100644
--- a/pc/simulcast_description.h
+++ b/pc/simulcast_description.h
@@ -8,8 +8,8 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#ifndef PC_SIMULCASTDESCRIPTION_H_
-#define PC_SIMULCASTDESCRIPTION_H_
+#ifndef PC_SIMULCAST_DESCRIPTION_H_
+#define PC_SIMULCAST_DESCRIPTION_H_
 
 #include <string>
 #include <vector>
@@ -108,4 +108,4 @@
 
 }  // namespace cricket
 
-#endif  // PC_SIMULCASTDESCRIPTION_H_
+#endif  // PC_SIMULCAST_DESCRIPTION_H_