Disable implicit fallthrough warning for zlib

https://logs.chromium.org/logs/crashpad/buildbucket/cr-buildbucket.appspot.com/8903960674082840512/+/steps/compile_with_ninja/0/stdout

I don't understand how this lingered for so long as I can't see what
changed. mini_chromium has had -Wimplicit-fallthough enabled for a long
time (https://chromium-review.googlesource.com/899847), and zlib DEPS
hasn't changed in a long time either
(https://chromium-review.googlesource.com/438585).

The most likely would presumably be that the compiler started
not-ignoring the enabling of the warning? But that doesn't seem too
likely either.

Bug: crashpad:309
Change-Id: I57480cc3f669ef94d0234afaf980281b7ddf8c51
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/1772205
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Joshua Peraza <jperaza@chromium.org>

DEPS

@@ -33,7 +33,7 @@ deps = {
       '8048ece6c16c91acfe0d36d1d3cc0890ab6e945c',
   'crashpad/third_party/mini_chromium/mini_chromium':
       Var('chromium_git') + '/chromium/mini_chromium@' +
-      '6ad086b2b6ed3b3169226ee9f311eb2332f332c2',
+      '5889767521eed1483b5858b12c54893f21d72fd0',
   'crashpad/third_party/libfuzzer/src':
       Var('chromium_git') + '/chromium/llvm-project/compiler-rt/lib/fuzzer.git@' +
       'fda403cf93ecb8792cb1d061564d89a6553ca020',

build/BUILDCONFIG.gn

@@ -55,6 +55,7 @@ declare_args() {
 _default_configs = [
   "//third_party/mini_chromium/mini_chromium/build:default",
   "//third_party/mini_chromium/mini_chromium/build:Wexit_time_destructors",
+  "//third_party/mini_chromium/mini_chromium/build:Wimplicit_fallthrough",
 ]
 
 if (crashpad_use_libfuzzer) {

third_party/zlib/BUILD.gn

@@ -104,6 +104,10 @@ if (zlib_source == "external") {
       ]
     }
 
+    configs -= [
+      "//third_party/mini_chromium/mini_chromium/build:Wimplicit_fallthrough",
+    ]
+
     if (current_cpu == "x86" || current_cpu == "x64") {
       sources += [
         "zlib/crc_folding.c",