Add fuzzer corpora.

webrtc/test/fuzzers/corpora/README

+This is a collection of corpora for various WebRTC fuzzers. To use
+them, the gn targets define seed_corpus=$corpus_dir, which causes the
+ClusterFuzz upload bot to bundle $corpus_dir and upload it.
+
+The format is simple: one file per test case. Specific notes are
+included below.
+
+### SDP ###
+This corpus is assembled manually from the following sources:

[pbos-webrtc, 2016/06/21 12:22:10]

Put some kind of "initially", here and below to indicate that the corpora can
have changed at the point this is being read.

[katrielc1, 2016/06/22 09:29:31]

Done.

+
+  - curl --silent https://www.ietf.org/rfc/rfc4317.txt | grep '^[ a-z]*=[^=]*$' | sed 's/^[[:space:]]*//' | awk -v RS='(^|\n)v=' '/./ {print "v="$0 > NR".sdp"}'
+  - all the SDPs used in the parser unit tests
+  - some manually gathered SDPs from Firefox and Opera
+
+The SDP tokens come from:
+
+ -  grep "^static const " webrtc/api/webrtcsdp.cc | cut -d'=' -f2 | cut -d ';' -f1 | tr -d '"' | tr -d "'" | tr -d ' ' | sort -u | grep -v '^(\n|\r|\r\n)$|^$' | sed -e 's/^/"/' -e 's/$/"/' | tail -n +2
+
+### STUN ###
+This corpus comprises the STUN packets used in the unit tests.
+
+### RT(C)P ###
+The RT(C)P corpora are assembled manually from unittests. RTCP was
+minimised first.
+
+There is also rt(c?)p-corpus-with-extra-byte, in which each sample is
+prefixed by the byte 0xff. Some of the rtp fuzzers need to decide
+which header extensions to enable, and the first byte of the fuzz data
+is used for this.