Revert "Run exo tests in parallel"

This reverts commit 245abec2.

Reason for revert: One of the exo tests is failing flakily now. See http://crbug.com/746227

Original change's description:
> Run exo tests in parallel
> 
> BUG=None
> TEST=All tests still should pass
> 
> Change-Id: I8a2cc5598baaa9692a33a79767f93869e5a75b9b
> Reviewed-on: https://chromium-review.googlesource.com/657622
> Reviewed-by: David Reveman <reveman@chromium.org>
> Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#500712}

TBR=reveman@chromium.org,oshima@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: None
Change-Id: I6a7d7bfef56b027a0ebf8b914f2967195029840b
Reviewed-on: https://chromium-review.googlesource.com/662842Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501215}

components/exo/test/run_all_unittests.cc

@@ -17,7 +17,7 @@ int main(int argc, char** argv) {
   mojo::edk::Init();
 #endif
 
-  return base::LaunchUnitTests(
+  return base::LaunchUnitTestsSerially(
       argc, argv,
       base::Bind(&ash::AshTestSuite::Run, base::Unretained(&test_suite)));
 }