Bug 1034081 - Never seek before startTime. r=rillian

Only adjust seek target up to startTime

--HG--
extra : rebase_source : 1154392a32c97b227f6293be06cbe1a31ab960e0

dom/media/webm/WebMReader.cpp

@@ -750,8 +750,16 @@ nsresult WebMReader::SeekInternal(int64_t aTarget)
   uint64_t target = aTarget * NS_PER_USEC;
 
   if (mSeekPreroll) {
-    target = std::max(uint64_t(StartTime() * NS_PER_USEC),
-                      target - mSeekPreroll);
+    uint64_t startTime = uint64_t(StartTime()) * NS_PER_USEC;
+    if (target < mSeekPreroll || target - mSeekPreroll < startTime) {
+      target = startTime;
+    } else {
+      target -= mSeekPreroll;
+    }
+    LOG(LogLevel::Debug,
+        ("Reader [%p] SeekPreroll: %f StartTime: %f AdjustedTarget: %f",
+        this, double(mSeekPreroll) / NS_PER_S,
+        double(startTime) / NS_PER_S, double(target) / NS_PER_S));
   }
   int r = nestegg_track_seek(mContext, trackToSeek, target);
   if (r != 0) {