Run angle_end2end_tests in batches

An unknown leak is causing OOM failures on the Intel/Windows bot. While
that's being investigated, this change unblocks developers by making
sure the tests are done in batches and memory is reclaimed occasionally.

Bug: angleproject:3153
No-Try: True
Change-Id: Ia03e8996554b308e0bd8a59d13bffc8bde3fbfba
Reviewed-on: https://chromium-review.googlesource.com/c/1481854
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635377}

testing/buildbot/test_suites.pyl

@@ -3057,7 +3057,10 @@
           '--use-gpu-in-tests',
           # ANGLE test retries deliberately disabled to prevent flakiness.
           # http://crbug.com/669196
-          '--test-launcher-retry-limit=0'
+          '--test-launcher-retry-limit=0',
+          # Work around leaks leading to out-of-memory as the test size grows.
+          # http://anglebug.com/3153
+          '--test-launcher-batch-limit=400'
         ],
         'should_retry_with_patch': False,
         'swarming': {