Add more nullptr check.

In some edge cases, these function may return nullptr which will crash
chrome.

TEST=passed unit tests and browser tests
BUG=1121330

Change-Id: I4863ac4a43b5dcdae684f948abbf40d627c4ab9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462745Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Jing Wang <jiwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815444}

chrome/browser/chromeos/input_method/assistive_suggester.cc

@@ -162,7 +162,9 @@ bool IsInternalWebsite(GURL url) {
 template <size_t N>
 bool IsAllowedUrl(const char* (&allowedDomains)[N]) {
   Browser* browser = chrome::FindLastActive();
-  if (browser && browser->window()->IsActive()) {
+  if (browser && browser->window() && browser->window()->IsActive() &&
+      browser->tab_strip_model() &&
+      browser->tab_strip_model()->GetActiveWebContents()) {
     GURL url = browser->tab_strip_model()
                    ->GetActiveWebContents()
                    ->GetLastCommittedURL();