Revert warning change that broke official builds

Cr-Commit-Position: refs/heads/master@{#305114}

build/common.gypi

@@ -5417,6 +5417,7 @@
           # removed as code is fixed.
           4100, # Unreferenced formal parameter
           4121, # Alignment of a member was sensitive to packing
+          4189, # Local variable is initialized but not referenced
           4244, # Conversion from 'type1' to 'type2', possible loss of data
           4481, # Nonstandard extension used: override specifier 'keyword'
           4505, # Unreferenced local function has been removed

build/config/compiler/BUILD.gn

@@ -691,6 +691,7 @@ config("default_warnings") {
 
       "/wd4100",  # Unreferenced formal function parameter.
       "/wd4121",  # Alignment of a member was sensitive to packing.
+      "/wd4189",  # A variable was declared and initialized but never used.
       "/wd4244",  # Conversion: possible loss of data.
       "/wd4481",  # Nonstandard extension: override specifier.
       "/wd4505",  # Unreferenced local function has been removed.