Remove IS_IOS checks in chrome/

The directory is not used on iOS at all.

Review-Url: https://codereview.chromium.org/2944973002
Cr-Commit-Position: refs/heads/master@{#480991}
diff --git a/chrome/browser/chrome_browser_main.h b/chrome/browser/chrome_browser_main.h
index f4cfa59c..b00bac9 100644
--- a/chrome/browser/chrome_browser_main.h
+++ b/chrome/browser/chrome_browser_main.h
@@ -145,7 +145,7 @@
 
   ChromeBrowserFieldTrials browser_field_trials_;
 
-#if !defined(OS_ANDROID) && !defined(OS_IOS)
+#if !defined(OS_ANDROID)
   std::unique_ptr<WebUsbDetector> web_usb_detector_;
 #endif