vs2015: avoid narrowing warning in plugin_service_impl

[171->263/443 ~9] CXX obj\content\browser\content_browser.plugin_service_impl.obj
FAILED: ninja -t msvc -e environment.x86 -- "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin\cl.exe" /nologo /showIncludes /FC @obj\content\browser\content_browser.plugin_service_impl.obj.rsp /c ..\..\content\browser\plugin_service_impl.cc /Foobj\content\browser\content_browser.plugin_service_impl.obj /Fdobj\content\content_browser.cc.pdb
d:\src\cr2\src\content\browser\plugin_service_impl.cc(842): error C2220: warning treated as error - no 'object' file generated
d:\src\cr2\src\content\browser\plugin_service_impl.cc(842): warning C4302: 'reinterpret_cast': truncation from 'HANDLE' to 'ATOM'

R=jam@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/1111913002

Cr-Commit-Position: refs/heads/master@{#327403}

content/browser/plugin_service_impl.cc

@@ -838,8 +838,8 @@ void PluginServiceImpl::AppActivated() {
 bool GetPluginPropertyFromWindow(
     HWND window, const wchar_t* plugin_atom_property,
     base::string16* plugin_property) {
-  ATOM plugin_atom = reinterpret_cast<ATOM>(
-      GetPropW(window, plugin_atom_property));
+  ATOM plugin_atom = static_cast<ATOM>(
+      reinterpret_cast<uintptr_t>(GetPropW(window, plugin_atom_property)));
   if (plugin_atom != 0) {
     WCHAR plugin_property_local[MAX_PATH] = {0};
     GlobalGetAtomNameW(plugin_atom,