Reland: Add mock_rtc_event_log.h.

Bug: webrtc:7642
Change-Id: I3f97a8b603e34819e1563b335c1eeb51f89b11ac
Reviewed-on: https://webrtc-review.googlesource.com/37081
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21480}

logging/BUILD.gn

@@ -235,6 +235,7 @@ if (rtc_enable_protobuf) {
       }
     }
   }
+
   if (rtc_include_tests) {
     rtc_executable("rtc_event_log2text") {
       testonly = true
@@ -283,3 +284,16 @@ if (rtc_enable_protobuf) {
     }
   }
 }
+
+if (rtc_include_tests) {
+  rtc_source_set("mocks") {
+    testonly = true
+    sources = [
+      "rtc_event_log/mock/mock_rtc_event_log.h",
+    ]
+    deps = [
+      ":rtc_event_log_api",
+      "../../test:test_support",
+    ]
+  }
+}