[Cleanup] Add missing #include. Remove useless ones.

This CL is the result of running include-what-you-use tool on part
of the code base (audio target and dependencies) plus manual fixes.

bug: webrtc:8311
Change-Id: I277d281ce943c3ecc1bd45fd8d83055931743604
Reviewed-on: https://webrtc-review.googlesource.com/c/106280
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25311}

rtc_base/helpers.h

@@ -11,6 +11,8 @@
 #ifndef RTC_BASE_HELPERS_H_
 #define RTC_BASE_HELPERS_H_
 
+#include <stddef.h>
+#include <stdint.h>
 #include <string>
 
 namespace rtc {