Revert "Fix to remove /wd4018 (-Wno-sign-compare) for /build/config"

This reverts commit 1f75f99f.

Reason for revert: This breaks compile on "Google Chrome Win" builder due to a integer sign mismatch in incompatible_applications_updater_win.cc(289,32).

Original change's description:
> Fix to remove /wd4018 (-Wno-sign-compare) for /build/config
> 
> This is a part of effort to remove /wd4018 warning suppression.
> 
> Master CL is
> https://chromium-review.googlesource.com/c/chromium/src/+/1152755
> 
> This CL was uploaded by git cl split.
> 
> R=​dpranke@chromium.org
> 
> Bug: 868248, 588506
> Change-Id: I3c31dc8f72266eda85904c4a860e156fadb2a76e
> Reviewed-on: https://chromium-review.googlesource.com/1152840
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#579311}

TBR=thakis@chromium.org,dpranke@chromium.org,pmonette@chromium.org,tikuta@chromium.org

Change-Id: Iaed7b5e6f2ff3c53e1846162ca8d14689f7ae5e1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 868248, 588506
Reviewed-on: https://chromium-review.googlesource.com/1156144Reviewed-by: Patti <patricialor@chromium.org>
Commit-Queue: Patti <patricialor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579312}

build/config/compiler/BUILD.gn

@@ -1302,6 +1302,7 @@ 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_metrics_recorder_win.h

@@ -37,7 +37,7 @@ class ThirdPartyMetricsRecorder : public ModuleDatabaseObserver {
   void AddUnsignedModuleToCrashkeys(const base::string16& module_basename);
 
   // The index of the crash key that is currently being updated.
-  size_t current_key_index_ = 0;
+  int current_key_index_ = 0;
 
   // The value of the crash key that is currently being updated.
   std::string current_value_;