Allow libffmpeg to be built for Chromium-based browsers
https://patchwork.ffmpeg.org/patch/4500/

NOTE: This patch has -U1 unified context to be less likely
to need rebasing across ffmpeg versions.

--- a/Makefile
+++ b/Makefile
@@ -43,2 +43,3 @@
 include $(SRC_PATH)/ffbuild/common.mak
+include $(SRC_PATH)/ffbuild/libffmpeg.mak
 
--- a/chromium.c
+++ b/chromium.c
@@ -0,0 +1,8 @@
+#include "libavformat/avformat.h"
+#include "libavformat/internal.h"
+
+int64_t av_stream_get_first_dts(const AVStream *st);
+int64_t av_stream_get_first_dts(const AVStream *st)
+{
+    return cffstream(st)->first_dts;
+}
--- a/ffbuild/libffmpeg.mak
+++ b/ffbuild/libffmpeg.mak
@@ -0,0 +1,27 @@
+LIBFFMPEG = $(SLIBPREF)ffmpeg$(SLIBSUF)
+LIBFFMPEG_LINK = $(LD) -shared -Wl,-soname,$(LIBFFMPEG) -Wl,-Bsymbolic -Wl,-z,now -Wl,-z,relro -Wl,-z,defs -Wl,--gc-sections $(LDFLAGS) $(LDLIBFLAGS) -o $(LIBFFMPEG)
+
+ifeq ($(CONFIG_SHARED),yes)
+LIBFFMPEG_DEPS = libavcodec/$(SLIBPREF)avcodec$(SLIBSUF) libavformat/$(SLIBPREF)avformat$(SLIBSUF) libavutil/$(SLIBPREF)avutil$(SLIBSUF)
+else
+LIBFFMPEG_DEPS = libavcodec/$(LIBPREF)avcodec$(LIBSUF) libavformat/$(LIBPREF)avformat$(LIBSUF) libavutil/$(LIBPREF)avutil$(LIBSUF) libswresample/$(LIBPREF)swresample$(LIBSUF)
+endif
+
+$(LIBFFMPEG): $(LIBFFMPEG_DEPS) chromium.o
+ifeq ($(CONFIG_SHARED),yes)
+	$(LIBFFMPEG_LINK) -Wl,--no-as-needed chromium.o -lavcodec -lavformat -lavutil
+else
+	$(LIBFFMPEG_LINK) -Wl,--whole-archive $^ -Wl,--no-whole-archive $(EXTRALIBS-avcodec) $(EXTRALIBS-avformat) $(EXTRALIBS-avutil) $(EXTRALIBS-swresample)
+endif
+
+libffmpeg: $(LIBFFMPEG)
+
+install-libffmpeg: $(LIBFFMPEG)
+	$(Q)mkdir -p "$(SHLIBDIR)/chromium"
+	$(INSTALL) -m 755 $< "$(SHLIBDIR)/chromium/$<"
+	$(STRIP) "$(SHLIBDIR)/chromium/$<"
+
+uninstall-libffmpeg:
+	$(RM) "$(SHLIBDIR)/chromium/$(LIBFFMPEG)"
+
+.PHONY: libffmpeg install-libffmpeg uninstall-libffmpeg