Try to add more sanity checking to help track down a crash....

chrome/browser/autocomplete/autocomplete.cc

 // Copyright (c) 2009 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.
 
 #include "chrome/browser/autocomplete/autocomplete.h"
 
 #include <algorithm>
 
 #include "app/l10n_util.h"
 #include "base/basictypes.h"
@@ skipping 493 matching lines @@
 
 // static
 size_t AutocompleteProvider::max_matches_ = 3;
 
 AutocompleteProvider::~AutocompleteProvider() {
   Stop();
 }
 
 void AutocompleteProvider::SetProfile(Profile* profile) {
   DCHECK(profile);
-  Stop();  // It makes no sense to continue running a query from an old profile.
+  DCHECK(done_);  // The controller should have already stopped us.
   profile_ = profile;
 }
 
 // static
 size_t AutocompleteProvider::TrimHttpPrefix(std::wstring* url) {
   url_parse::Component scheme;
   if (!url_util::FindAndCompareScheme(WideToUTF8(*url), chrome::kHttpScheme,
                                       &scheme))
     return 0;  // Not "http".
 
@@ skipping 156 matching lines @@
 }
 
 AutocompleteController::~AutocompleteController() {
   for (ACProviders::iterator i(providers_.begin()); i != providers_.end(); ++i)
     (*i)->Release();
 
   providers_.clear();  // Not really necessary.
 }
 
 void AutocompleteController::SetProfile(Profile* profile) {
+  Stop(true);
   for (ACProviders::iterator i(providers_.begin()); i != providers_.end(); ++i)
     (*i)->SetProfile(profile);
   input_.Clear();  // Ensure we don't try to do a "minimal_changes" query on a
                    // different profile.
 }
 
 void AutocompleteController::Start(const std::wstring& text,
                                    const std::wstring& desired_tld,
                                    bool prevent_inline_autocomplete,
                                    bool prefer_keyword,
@@ skipping 48 matching lines @@
   for (ACProviders::const_iterator i(providers_.begin()); i != providers_.end();
        ++i) {
     if (!(*i)->done())
       (*i)->Stop();
   }
 
   update_delay_timer_.Stop();
   updated_latest_result_ = false;
   delay_interval_has_passed_ = false;
   done_ = true;
-  if (clear_result) {
+  if (clear_result && !result_.empty()) {
     result_.Reset();
     NotificationService::current()->Notify(
         NotificationType::AUTOCOMPLETE_CONTROLLER_RESULT_UPDATED,
         Source<AutocompleteController>(this),
         Details<const AutocompleteResult>(&result_));
     // NOTE: We don't notify AUTOCOMPLETE_CONTROLLER_DEFAULT_MATCH_UPDATED since
     // we're trying to only clear the popup, not touch the edit... this is all
     // a mess and should be cleaned up :(
   }
   latest_result_.CopyFrom(result_);
@@ skipping 192 matching lines @@
 void AutocompleteController::CheckIfDone() {
   for (ACProviders::const_iterator i(providers_.begin()); i != providers_.end();
        ++i) {
     if (!(*i)->done()) {
       done_ = false;
       return;
     }
   }
   done_ = true;
 }