GN: fixes for the official build.
The C++ define OFFICIAL_BUILD wasn't getting set, and the widevine version .rc files was getting linked twice.
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg
R=dpranke
TBR=ddorwin@chromium.org
Review URL: https://codereview.chromium.org/
1134623003
Cr-Commit-Position: refs/heads/master@{#329308}