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

This is a reland of 1f75f99f

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}

Bug: 868248, 588506
Change-Id: Ic673677dfc0800b361d97f6516ee61887635d9d4
Reviewed-on: https://chromium-review.googlesource.com/1156164
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579401}

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/module_info_win.h

@@ -15,7 +15,7 @@
 // maintain information about a module, usually in a std::map.
 
 // Used by the ModuleDatabase as a unique identifier for a module.
-using ModuleId = int;
+using ModuleId = uint32_t;
 
 // This is the constant portion of the module information, and is used to
 // uniquely identify one.

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.
-  int current_key_index_ = 0;
+  size_t current_key_index_ = 0;
 
   // The value of the crash key that is currently being updated.
   std::string current_value_;