ChromeLauncherControllerArcDefaultAppsTest.DefaultApps TearDown icon crash.

This came up while testing ChromeLauncherControllerImpl more like production use:
  https://codereview.chromium.org/2804913002
I added a workaround to clear the icon loaders for testing.

Here is where the first tasks are posted (LoadForScaleFactor is called 4 times per test):
#2 0x00000812a3db ArcAppIcon::LoadForScaleFactor()
#3 0x00000812a0a9 ArcAppIcon::Source::GetImageForScale()
#4 0x7fe9c1d0ad3b gfx::internal::ImageSkiaStorage::FindRepresentation()
#5 0x7fe9c1d0ccb4 gfx::ImageSkia::EnsureRepsForSupportedScales()
#6 0x000008131e02 ArcAppIconLoader::FetchImage()
#7 0x000007f1696a ChromeLauncherControllerImpl::InsertAppLauncherItem()
#8 0x000007f16589 ChromeLauncherControllerImpl::CreateAppLauncherItem()
#9 0x000008160bd3 ArcAppDeferredLauncherController::RegisterDeferredLaunch()
#10 0x000008150476 arc::LaunchApp()
#11 0x00000815081e arc::LaunchApp()
#12 0x000001b35fa4 ChromeLauncherControllerArcDefaultAppsTest_DefaultApps_Test::TestBody()

The next tasks are posted to invoke DecodeImage when spinning a run loop (during TearDown):
[22466:22466:0406/154730.719793:4490434140084:ERROR:arc_app_utils.cc(72)] Failed to prioritize ARC
[22466:22466:0406/154730.724729:4490434145014:ERROR:image_decoder.cc(176)] MSW DecodeImage
#0 0x7fe9c2953afb base::debug::StackTrace::StackTrace()
#1 0x7fe9c295283c base::debug::StackTrace::StackTrace()
#2 0x000004e847d0 ImageDecoder::StartWithOptionsImpl()
#3 0x000004e8439b ImageDecoder::StartWithOptions()
#4 0x000004e84316 ImageDecoder::Start()
#5 0x000004e84410 ImageDecoder::Start()
#6 0x00000812c174 ArcAppIcon::OnIconRead()

Then finally we get this crash as the run loop continues:
Received signal 11 SEGV_MAPERR 000000000000
#0 0x7fe9c2953afb base::debug::StackTrace::StackTrace()
#1 0x7fe9c295283c base::debug::StackTrace::StackTrace()
#2 0x7fe9c295360f base::debug::(anonymous namespace)::StackDumpSignalHandler()
#3 0x7fe9c6398330 <unknown>
#4 0x000004e85368 (anonymous namespace)::BindToBrowserConnector()
#5 0x000004e8505f (anonymous namespace)::DecodeImage()