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

diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 7b45a85..091037c 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -1122,6 +1122,8 @@
     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)