Fix -Wformat error in Win-Clang build

rtc::PlatformThreadId is pid_t (32-bit signed int) on Linux and Mac,
but DWORD (32-bit unsigned int) on Windows.

Using the %d printf specifier is therefore not correct on Windows,
and Clang would warn about it:

..\..\third_party\webrtc\base\event_tracer.cc(124,46) :  error: format specifies
type 'int' but the argument has type 'rtc::PlatformThreadId' (aka 'unsigned
long') [-Werror,-Wformat]
                e.phase, e.timestamp, e.pid, e.tid);
                                             ^~~~~

This commit fixes the problem by explicitly casting to int before printing.

BUG=82385

Review URL: https://codereview.webrtc.org/1514253002 .

Cr-Commit-Position: refs/heads/master@{#10982}

webrtc/base/event_tracer.cc

@@ -121,7 +121,7 @@ class EventLogger final {
                 ", \"pid\": %d"
                 ", \"tid\": %d}\n",
                 has_logged_event ? "," : " ", e.name, e.category_enabled,
-                e.phase, e.timestamp, e.pid, e.tid);
+                e.phase, e.timestamp, static_cast<int>(e.pid), e.tid);
         has_logged_event = true;
       }
       if (shutting_down)