Add transition define RTC_OBJC_HAVE_LEGACY_RTC_RTP_FRAGMENTATION_HEADER

Also tweak presubmit checks to allow changes to RTCMacros.h.

Bug: webrtc:6471
No-Presubmit: True
Change-Id: I19e38e4cb05b831ebd2faa223029f36d45f480ff
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/193621
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32619}

PRESUBMIT.py

@@ -1122,6 +1122,8 @@ def CheckObjcApiSymbols(input_api, output_api, source_file_filter):
     for f in input_api.AffectedSourceFiles(file_filter):
         if not f.LocalPath().endswith('.h') or not 'sdk/objc' in f.LocalPath():
             continue
+        if f.LocalPath().endswith('sdk/objc/base/RTCMacros.h'):
+            continue
         contents = input_api.ReadFile(f)
         for match in rtc_objc_export.finditer(contents):
             export_block = match.group(0)

sdk/objc/base/RTCMacros.h

@@ -49,4 +49,9 @@
 #define RTC_FWD_DECL_OBJC_CLASS(classname) typedef struct objc_object classname
 #endif
 
+// TODO(bugs.webrtc.org/6471): This define is a transition aid. It will be
+// deleted together with the RTCRtpFragmentationHeader argument to
+// RTCVideoEncoderCallback, see RTCVideoEncoder.h.
+#define RTC_OBJC_HAVE_LEGACY_RTC_RTP_FRAGMENTATION_HEADER 1
+
 #endif  // SDK_OBJC_BASE_RTCMACROS_H_