clang/win: Fix -Wsign-compare warning after https://codereview.chromium.org/1631903002

Also turn on chromium_code for these targets in gyp.  This makes warnings
for these targets consistent between gyp and gn.

BUG=550983, 581133, 82385
TBR=grt@chromium.org

Review URL: https://codereview.chromium.org/1637793003

Cr-Commit-Position: refs/heads/master@{#371520}

chrome/installer/test/alternate_version_generator.cc

@@ -425,8 +425,8 @@ bool UpdateManifestVersion(const base::FilePath& manifest,
     return false;
   }
   DCHECK(modified);
-  return base::WriteFile(manifest, &contents[0], contents.size()) ==
-         contents.size();
+  int written = base::WriteFile(manifest, &contents[0], contents.size());
+  return written != -1 && static_cast<size_t>(written) == contents.size();
 }
 
 bool IncrementNewVersion(upgrade_test::Direction direction,

chrome/installer/upgrade_test.gyp

@@ -4,6 +4,7 @@
 
 {
   'variables': {
+    'chromium_code': 1,
     'branding_dir': '../app/theme/<(branding_path_component)',
     'version_py': '<(DEPTH)/build/util/version.py',
     'version_path': '../../chrome/VERSION',