Added trace event to screenrecord

Bug: 135278534
Test: Run Perfetto with "screen recording" enabled and execute
a query for the "first_frame" slice

MergedIn: Ifb984bfb0cdf48c61352e2923de42ce0ca06ace7
Change-Id: I20abf7300e8c46b6e6dc52096db699e95c93297d

cmds/screenrecord/screenrecord.cpp

@@ -417,6 +417,7 @@ static status_t runEncoder(const sp<MediaCodec>& encoder,
     int64_t endWhenNsec = startWhenNsec + seconds_to_nanoseconds(gTimeLimitSec);
     DisplayInfo mainDpyInfo;
     Vector<int64_t> timestamps;
+    bool firstFrame = true;
 
     assert((rawFp == NULL && muxer != NULL) || (rawFp != NULL && muxer == NULL));
 
@@ -433,6 +434,11 @@ static status_t runEncoder(const sp<MediaCodec>& encoder,
         int64_t ptsUsec;
         uint32_t flags;
 
+        if (firstFrame) {
+            ATRACE_NAME("first_frame");
+            firstFrame = false;
+        }
+
         if (systemTime(CLOCK_MONOTONIC) > endWhenNsec) {
             if (gVerbose) {
                 printf("Time limit reached\n");