Don't include nacl_irt_x86_64.nexe in isolate for 32 bit Linux builds which don't have it.

This is breaking after r295514 which enforces that every file listed in an isolate file exists.

BUG=414808
R=erg@chromium.org

Review URL: https://codereview.chromium.org/584553006

Cr-Commit-Position: refs/heads/master@{#295548}

chrome/chrome.isolate

@@ -30,13 +30,19 @@
           '<(PRODUCT_DIR)/chrome_100_percent.pak',
           '<(PRODUCT_DIR)/locales/en-US.pak',
           '<(PRODUCT_DIR)/locales/fr.pak',
-          '<(PRODUCT_DIR)/nacl_irt_x86_64.nexe',
         ],
         'isolate_dependency_untracked': [
           '<(PRODUCT_DIR)/resources/extension/',
         ],
       },
     }],
+    ['OS=="win" or (OS=="linux" and target_arch=="x64")', {
+      'variables': {
+        'isolate_dependency_tracked': [
+          '<(PRODUCT_DIR)/nacl_irt_x86_64.nexe',
+        ],
+      },
+    }],
     ['OS=="mac"', {
       'variables': {
         'command': [