Enable -Wshift-negative-value.

base/allocator/BUILD.gn

Index: base/allocator/BUILD.gn
diff --git a/base/allocator/BUILD.gn b/base/allocator/BUILD.gn
index 8cdb06161f5c23afb34058f71a449024c5a93d54..4fe226c9b8bbcda4e2dc372e0a23e84329a19d89 100644
--- a/base/allocator/BUILD.gn
+++ b/base/allocator/BUILD.gn
@@ -72,6 +72,10 @@ config("tcmalloc_flags") {
       # for magic2_ in debugallocation.cc (only built in Debug builds)
       # typedefs.
       "-Wno-unused-private-field",
+
+      # base/basictypes.h's kint64min definition shifts a negative value around,
+      # which is undefined behavior. TODO(thakis): Try to get that fixed.
+      "-Wno-shift-negative-value",
     ]
   } else {
     cflags = []