Add a official build target for Linux, builds with OFFICIAL_BUILD.

build/SConscript.main

Index: build/SConscript.main
diff --git a/build/SConscript.main b/build/SConscript.main
index fa4483ae95ac3b99f853604056c410887b12c790..35ee04efc8852691fee704130ea0b3a50263c6d9 100644
--- a/build/SConscript.main
+++ b/build/SConscript.main
@@ -504,6 +504,11 @@ linux_env.Append(
 
     ICU_LIBS = ['icu'],
 )
+
+# Build an "official" build (DCHECKs completely compiled out, etc).
+if ARGUMENTS.get('OFFICIAL') == '1':
+  linux_env.Append(CPPDEFINES=['OFFICIAL_BUILD'])
+
 # Build with support for gcov when COVERAGE=1.
 if ARGUMENTS.get('COVERAGE') == '1':
   linux_env.Append(CCFLAGS=['-fprofile-arcs', '-ftest-coverage'])