(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}

rtc_base/numerics/math_utils.h

@@ -8,8 +8,8 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#ifndef RTC_BASE_NUMERICS_MATHUTILS_H_
-#define RTC_BASE_NUMERICS_MATHUTILS_H_
+#ifndef RTC_BASE_NUMERICS_MATH_UTILS_H_
+#define RTC_BASE_NUMERICS_MATH_UTILS_H_
 
 #include <math.h>
 #include <type_traits>
@@ -36,4 +36,4 @@ typename std::make_unsigned<T>::type unsigned_difference(T x, T y) {
   return static_cast<unsigned_type>(x) - static_cast<unsigned_type>(y);
 }
 
-#endif  // RTC_BASE_NUMERICS_MATHUTILS_H_
+#endif  // RTC_BASE_NUMERICS_MATH_UTILS_H_