Handle destroyed tab cases in Reader Mode

There are instances where callbacks and events can be triggered
and then the host tab for Reader Mode is destroyed. This change
adds checks for these cases.

BUG=556930, 556456, 556354

Review URL: https://codereview.chromium.org/1451103004

Cr-Commit-Position: refs/heads/master@{#361168}

chrome/android/java/src/org/chromium/chrome/browser/dom_distiller/ReaderModeManager.java

@@ -299,9 +299,14 @@ public class ReaderModeManager extends TabModelSelectorTabObserver
                     boolean isMainFrame, String validatedUrl, boolean isErrorPage,
                     boolean isIframeSrcdoc) {
                 if (!isMainFrame) return;
-                mTabStatusMap.get(readerTabId).setUrl(validatedUrl);
+
+                // Make sure the tab was not destroyed.
+                ReaderModeTabInfo tabInfo = mTabStatusMap.get(readerTabId);
+                if (tabInfo == null) return;
+
+                tabInfo.setUrl(validatedUrl);
                 if (DomDistillerUrlUtils.isDistilledPage(validatedUrl)) {
-                    mTabStatusMap.get(readerTabId).setStatus(STARTED);
+                    tabInfo.setStatus(STARTED);
                     closeReaderPanel(StateChangeReason.UNKNOWN, true);
                     mReaderModePageUrl = validatedUrl;
                 }
@@ -316,16 +321,20 @@ public class ReaderModeManager extends TabModelSelectorTabObserver
                 if (isNavigationInPage) return;
                 if (DomDistillerUrlUtils.isDistilledPage(url)) return;
 
-                mTabStatusMap.get(readerTabId).setStatus(POSSIBLE);
+                // Make sure the tab was not destroyed.
+                ReaderModeTabInfo tabInfo = mTabStatusMap.get(readerTabId);
+                if (tabInfo == null) return;
+
+                tabInfo.setStatus(POSSIBLE);
                 if (!TextUtils.equals(url,
                         DomDistillerUrlUtils.getOriginalUrlFromDistillerUrl(
                                 mReaderModePageUrl))) {
-                    mTabStatusMap.get(readerTabId).setStatus(NOT_POSSIBLE);
+                    tabInfo.setStatus(NOT_POSSIBLE);
                     mIsUmaRecorded = false;
                 }
                 mReaderModePageUrl = null;
-                if (mTabStatusMap.containsKey(readerTabId)
-                        && mTabStatusMap.get(readerTabId).getStatus() != POSSIBLE) {
+
+                if (tabInfo.getStatus() != POSSIBLE) {
                     closeReaderPanel(StateChangeReason.UNKNOWN, false);
                 } else {
                     requestReaderPanelShow(StateChangeReason.UNKNOWN);
@@ -409,10 +418,14 @@ public class ReaderModeManager extends TabModelSelectorTabObserver
                 new DistillablePageUtils.PageDistillableDelegate() {
                     @Override
                     public void onIsPageDistillableResult(boolean isDistillable, boolean isLast) {
-                        if (!mTabStatusMap.containsKey(readerTabId)) return;
                         ReaderModeTabInfo tabInfo = mTabStatusMap.get(readerTabId);
-
                         Tab readerTab = mTabModelSelector.getTabById(readerTabId);
+
+                        // It is possible that the tab was destroyed before this callback happens.
+                        // TODO(wychen/mdjones): Remove the callback when a Tab/WebContents is
+                        // destroyed so that this never happens.
+                        if (readerTab == null || tabInfo == null) return;
+
                         // Make sure the page didn't navigate while waiting for a response.
                         if (!readerTab.getUrl().equals(tabInfo.getUrl())) return;