Added per-user external extensions directory on Linux Chromium builds.

On Chromium builds on Linux, External Extensions will also be installed from
DIR_USER_EXTERNAL_EXTENSIONS.

BUG=513881

Review URL: https://codereview.chromium.org/1256863003

Cr-Commit-Position: refs/heads/master@{#341178}

chrome/browser/extensions/external_provider_impl.cc

@@ -611,7 +611,7 @@ void ExternalProviderImpl::CreateExternalProviders(
                 bundled_extension_creation_flags)));
 
     // Define a per-user source of external extensions.
-#if defined(OS_MACOSX)
+#if defined(OS_MACOSX) || (defined(OS_LINUX) && defined(CHROMIUM_BUILD))
     provider_list->push_back(
         linked_ptr<ExternalProviderInterface>(
             new ExternalProviderImpl(

chrome/common/chrome_paths.cc

@@ -481,7 +481,8 @@ bool PathProvider(int key, base::FilePath* result) {
       break;
     }
 #endif
-#if defined(OS_CHROMEOS) || (defined(OS_MACOSX) && !defined(OS_IOS))
+#if defined(OS_CHROMEOS) || (defined(OS_LINUX) && defined(CHROMIUM_BUILD)) || \
+    (defined(OS_MACOSX) && !defined(OS_IOS))
     case chrome::DIR_USER_EXTERNAL_EXTENSIONS: {
       if (!PathService::Get(chrome::DIR_USER_DATA, &cur))
         return false;

chrome/common/chrome_paths.h

@@ -50,11 +50,13 @@ enum {
   DIR_USER_APPLICATIONS,        // ~/Applications
   DIR_USER_LIBRARY,             // ~/Library
 #endif
-#if defined(OS_CHROMEOS) || (defined(OS_MACOSX) && !defined(OS_IOS))
+#if defined(OS_CHROMEOS) || (defined(OS_LINUX) && defined(CHROMIUM_BUILD)) || \
+    (defined(OS_MACOSX) && !defined(OS_IOS))
   DIR_USER_EXTERNAL_EXTENSIONS,  // Directory for per-user external extensions
-                                 // on Chrome Mac.  On Chrome OS, this path is
-                                 // used for OEM customization.
-                                 // Getting this path does not create it.
+                                 // on Chrome Mac and Chromium Linux.
+                                 // On Chrome OS, this path is used for OEM
+                                 // customization. Getting this path does not
+                                 // create it.
 #endif
 
 #if defined(OS_LINUX)