Make PNaCl component downloadable for chromeos-chrome built for Linux

Chrome4ChromeOS for Linux packages needs pNaCl but it is not part of package
so make it downloadable when running on Linux. In Chrome OS PNaCl is part of
rootfs.

BUG=422121
TEST=manual

Review URL: https://codereview.chromium.org/662713006

Cr-Commit-Position: refs/heads/master@{#300236}

chrome/browser/chrome_browser_main.cc

@@ -402,9 +402,16 @@ void RegisterComponentsForUpdate() {
   RegisterPepperFlashComponent(cus);
   RegisterSwiftShaderComponent(cus);
   RegisterWidevineCdmComponent(cus);
-#if !defined(DISABLE_NACL)
-  g_browser_process->pnacl_component_installer()->RegisterPnaclComponent(cus);
 #endif
+
+#if !defined(DISABLE_NACL) && !defined(OS_ANDROID)
+#if defined(OS_CHROMEOS)
+  // PNaCl on Chrome OS is on rootfs and there is no need to download it. But
+  // Chrome4ChromeOS on Linux doesn't contain PNaCl so enable component
+  // installer when ruining on Linux. See crbug.com/422121 for more details.
+  if (!base::SysInfo::IsRunningOnChromeOS())
+#endif
+    g_browser_process->pnacl_component_installer()->RegisterPnaclComponent(cus);
 #endif
 
   if (translate::CldDataSource::ShouldRegisterForComponentUpdates()) {