Use ByteStream in downloads system to decouple source and sink.

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 # running overnight and removing outdated bugs/suppressions.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 1286 matching lines @@
 }
 {
    bug_93006d
    Heapcheck:Leak
    ...
    fun:DownloadManager::AddObserver
    fun:*SelectFileObserver
    fun:DownloadManagerTest_*
 }
 {
-   bug_93006e
-   Heapcheck:Leak
-   ...
-   fun:DownloadResourceHandler::OnWillRead
-   fun:BufferedResourceHandler::UseAlternateResourceHandler
-   fun:BufferedResourceHandler::CompleteResponseStarted
-   fun:BufferedResourceHandler::OnReadCompleted
-   fun:ResourceDispatcherHost::CompleteRead
-   fun:ResourceDispatcherHost::OnReadCompleted
-   fun:net::URLRequest::NotifyReadCompleted
-   fun:net::URLRequestJob::NotifyReadComplete
-   fun:net::URLRequestTestJob::ProcessNextOperation
-   fun:net::URLRequestTestJob::ProcessOnePendingMessage
-   fun:ResourceDispatcherHostTest_*
-}
-{
    bug_94195
    Heapcheck:Leak
    ...
    fun:GpuBlacklist::LoadGpuBlacklist
 }
 {
    bug_94764
    Heapcheck:Leak
    fun:remoting::ClientSession::UnpressKeys
    fun:remoting::ClientSessionTest_UnpressKeys_Test::TestBody
@@ skipping 523 matching lines @@
    fun:RefcountedProfileKeyedServiceFactory::SetTestingFactoryAndUse
    fun:TestingProfile::CreateWebDataService
 }
 {
    bug_129392
    Heapcheck:Leak
    fun:::ChromeTestSuiteInitializer::SetUpHostResolver
    fun:::ChromeTestSuiteInitializer::OnTestStart
    fun:testing::internal::TestEventRepeater::OnTestStart
 }