Add Heapchecker suppression for DownloadManagerTests, add Valgrind suppressions

tools/heapcheck/suppressions.txt

 # There are three kinds of suppressions in this file.
 # 1. third party stuff we have no control over
 #
 # 2. intentional unit test errors, or stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system (but a few aren't yet).
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 1702 matching lines @@
    fun:BrowserAccessibilityManagerTest_TestCreateEmptyDocument_Test::TestBody
 }
 {
    bug_91845_c
    Heapcheck:Leak
    ...
    fun:BrowserAccessibilityManager::Create
    fun:BrowserAccessibilityManager::CreateEmptyDocument
    fun:BrowserAccessibilityManagerTest_TestCreateEmptyDocument_Test::TestBody
 }
+{
+   bug_93006a
+   Heapcheck:Leak
+   fun:DownloadManager::Init
+   fun:DownloadManagerTest*
+   fun:DownloadManagerTest_*
+}
+{
+   bug_93006b
+   Heapcheck:Leak
+   fun:DownloadManagerTest*
+   fun:DownloadManagerTest_*
+}
+{
+   bug_93006c
+   Heapcheck:Leak
+   ...
+   fun:MockDownloadManager*
+   fun:DownloadManagerTest*
+   fun:DownloadManagerTest_*
+}
+{
+   bug_93006d
+   Heapcheck:Leak
+   ...
+   fun:DownloadManager::AddObserver
+   fun:::SelectFileObserver::SelectFileObserver
+   fun:DownloadManagerTest_*
+}