Remove wd4018 flags

I confirmed this can be built with is_chrome_branded=true and is_official_build=true.

Bug: 868248
Cq-Include-Trybots: master.tryserver.chromium.win:win_chrome_official
Change-Id: I6bfbd22c89dd4d3d655fce0db10448afb6cb2103
Reviewed-on: https://chromium-review.googlesource.com/1157946
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579762}

build/config/compiler/BUILD.gn

@@ -1302,7 +1302,6 @@ config("default_warnings") {
 
       # http://crbug.com/588506 - Conversion suppressions waiting on Clang
       # -Wconversion.
-      "/wd4018",  # 'expression' : signed/unsigned mismatch
       "/wd4245",  # 'conversion' : conversion from 'type1' to 'type2',
                   # signed/unsigned mismatch
 

chrome/browser/conflicts/third_party_blocking_browsertest.cc

@@ -198,5 +198,5 @@ IN_PROC_BROWSER_TEST_F(ThirdPartyBlockingBrowserTest,
             ReadModuleBlacklistCache(module_blacklist_cache_path, &metadata,
                                      &blacklisted_modules, &md5_digest));
 
-  EXPECT_GE(blacklisted_modules.size(), 1);
+  EXPECT_GE(blacklisted_modules.size(), 1u);
 }