Disable Download button if current tab is showing offline page already, or an error or interstitial.

chrome/android/java/src/org/chromium/chrome/browser/omnibox/LocationBarTablet.java

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.omnibox;
 
 import android.animation.Animator;
 import android.animation.AnimatorListenerAdapter;
 import android.animation.ObjectAnimator;
 import android.content.Context;
@@ skipping 554 matching lines @@
         return ChromeFeatureList.isEnabled("DownloadsUi") && shouldShowPageActionButtons()
                 && !tab.isIncognito();
     }
 
     private boolean isSaveOfflineButtonEnabled() {
         if (mToolbarDataProvider == null) return false;
         Tab tab = mToolbarDataProvider.getTab();
         if (tab == null) return false;
         boolean isChromeScheme = tab.getUrl().startsWith(UrlConstants.CHROME_SCHEME)
                 || tab.getUrl().startsWith(UrlConstants.CHROME_NATIVE_SCHEME);
-        return !isChromeScheme;
+        boolean isValidTab = !tab.isOfflinePage() && !tab.isShowingErrorPage()
+                             && !tab.isShowingInterstitialPage();
+        return !isChromeScheme && isValidTab;
     }
 
     private boolean shouldShowPageActionButtons() {
         if (!mNativeInitialized) return true;
 
         // If the new downloads UI isn't enabled, the only page action is the bookmark button. It
         // should be shown if the delete button isn't showing. If the download UI is enabled, there
         // are two actions, bookmark and save offline, and they should be shown if the omnibox isn't
         // focused.
         return (!shouldShowDeleteButton() && !ChromeFeatureList.isEnabled("DownloadsUi"))
                 || !(mUrlBar.hasFocus() || mUrlFocusChangeInProgress);
     }
 
     private boolean shouldShowMicButton() {
         // If the download UI is enabled, the mic button should be only be shown when the url bar
         // is focused.
         return isVoiceSearchEnabled() && mNativeInitialized
                 && (!ChromeFeatureList.isEnabled("DownloadsUi")
                         || (mUrlBar.hasFocus() || mUrlFocusChangeInProgress));
     }
 }