Remove CHECK around duplicate FLAG lookup.

It causes an asan initialization-order-fiasco in trying to read the
names of other globally constructed data:

==21449==ERROR: AddressSanitizer: initialization-order-fiasco on address 0x7f6f297bc5e8 at pc 0x7f6f26b332a7 bp 0x7ffd479f8cb0 sp 0x7ffd479f8ca8
READ of size 8 at 0x7f6f297bc5e8 thread T0
    #0 0x7f6f26b332a6 in name
webrtc/base/flags.h:83:38
    #1 0x7f6f26b332a6 in Lookup
webrtc/base/flags.cc:133
    #2 0x7f6f26b332a6 in rtc::FlagList::Register(rtc::Flag*)
webrtc/base/flags.cc:260
    #3 0x7f6f2529972b in __cxx_global_var_init.1

BUG=

Review-Url: https://codereview.webrtc.org/2110963004
Cr-Commit-Position: refs/heads/master@{#13479}

webrtc/base/flags.cc

@@ -257,8 +257,10 @@ int FlagList::SetFlagsFromCommandLine(int* argc, const char** argv,
 
 void FlagList::Register(Flag* flag) {
   assert(flag != NULL && strlen(flag->name()) > 0);
-  RTC_CHECK(!Lookup(flag->name())) << "flag " << flag->name()
-                                   << " declared twice";
+  // NOTE: Don't call Lookup() within Register because it accesses the name_
+  // of other flags in list_, and if the flags are coming from two different
+  // compilation units, the initialization order between them is undefined, and
+  // this will trigger an asan initialization-order-fiasco error.
   flag->next_ = list_;
   list_ = flag;
 }