Mark all bool conversion operators as explicit

An explicit bool conversion operator will still be used implicitly
when an expression appears in "bool context", e.g., as the condition
in an if statement, or as argument to logical operators. The
`explicit` annotation prevents conversion in other contexts, e.g.,
converting both a and b to bool in an expression like `a == b`.

Bug: None
Change-Id: I79ef35b1ea831e6011ae472900375ae8a3e617ab
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/250664
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35927}

logging/rtc_event_log/events/rtc_event_field_encoding_parser.h

@@ -36,7 +36,9 @@ class RtcEventLogParseStatus {
   }
 
   bool ok() const { return error_.empty(); }
-  ABSL_DEPRECATED("Use ok() instead") operator bool() const { return ok(); }
+  ABSL_DEPRECATED("Use ok() instead") explicit operator bool() const {
+    return ok();
+  }
 
   std::string message() const { return error_; }