Pin shell32.dll in ppapi processes

Due to an OS bug shell32.dll may get unloaded while still in use.
Pinning shell32.dll avoids this and the associated crashes with no
downside.

This is an expansion of the fix in crrev.com/c/2505835

Bug: 1076771
Change-Id: I89153998a5dabb345db0435877201de771a60b5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2588012
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836825}

content/ppapi_plugin/ppapi_plugin_main.cc

@@ -68,6 +68,11 @@ int PpapiPluginMain(const MainFunctionParams& parameters) {
   const base::CommandLine& command_line = parameters.command_line;
 
 #if defined(OS_WIN)
+  // https://crbug.com/1139752 Premature unload of shell32 caused process to
+  // crash during process shutdown.
+  HMODULE shell32_pin = ::LoadLibrary(L"shell32.dll");
+  UNREFERENCED_PARAMETER(shell32_pin);
+
   g_target_services = parameters.sandbox_info->target_services;
 #endif