Add mock_rtc_event_log.h.

Bug: webrtc:7642
Change-Id: Id3aa84d79e5e1a0520a968117cee550c9dd33c16
Reviewed-on: https://webrtc-review.googlesource.com/37040
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21475}

logging/BUILD.gn

@@ -235,6 +235,7 @@ if (rtc_enable_protobuf) {
       }
     }
   }
+
   if (rtc_include_tests) {
     rtc_executable("rtc_event_log2text") {
       testonly = true
@@ -283,3 +284,14 @@ if (rtc_enable_protobuf) {
     }
   }
 }
+
+rtc_source_set("mocks") {
+  testonly = true
+  sources = [
+    "rtc_event_log/mock/mock_rtc_event_log.h",
+  ]
+  deps = [
+    ":rtc_event_log_api",
+    "../../test:test_support",
+  ]
+}