experiment with -Wimplicit-fallthrough

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index dd999251dbad7a0367cabd9752e680b9cb70153d..18e261d64641ff3a26ba80664aff491423690409 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -2481,6 +2481,7 @@
             'cflags!': [
               '-Wall',
               '-Wextra',
+              '-Wimplicit-fallthrough',
             ],
             'cflags_cc': [
               # Don't warn about hash_map in third-party code.
@@ -2542,7 +2543,7 @@
           }],
           [ 'OS=="mac" or OS=="ios"', {
             'xcode_settings': {
-              'WARNING_CFLAGS!': ['-Wall', '-Wextra'],
+              'WARNING_CFLAGS!': ['-Wall', '-Wextra', '-Wimplicit-fallthrough'],
             },
             'conditions': [
               ['buildtype=="Official"', {
@@ -3306,6 +3307,7 @@
           ['clang==1', {
             'cflags': [
               '-Wheader-hygiene',
+              '-Wimplicit-fallthrough',
 
               # Don't die on dtoa code that uses a char as an array index.
               '-Wno-char-subscripts',
@@ -4010,6 +4012,7 @@
               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'WARNING_CFLAGS': [
                 '-Wheader-hygiene',
+                '-Wimplicit-fallthrough',
 
                 # This warns on using ints as initializers for floats in
                 # initializer lists (e.g. |int a = f(); CGSize s = { a, a };|),