Vulkan: Throttle when way ahead of device.

This CL adds a fence wait when we get more than 100 serials behind the
device. This fixes an ASSERT when unit tests or offscreen performance
tests get way ahead of the device.

Bug: angleproject:3630
Bug: angleproject:4281
Change-Id: I90f9af1b2ceb2b1cd9f2f638d6d84caaeeb83bb1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057351
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>

src/libANGLE/renderer/vulkan/ContextVk.cpp

@@ -459,14 +459,16 @@ angle::Result CommandQueue::submitFrame(vk::Context *context,
 
     mInFlightCommands.emplace_back(scopedBatch.release());
 
-    // CPU should be throttled to avoid mInFlightCommands from growing too fast.  That is done
-    // on swap() though, and there could be multiple submissions in between (through glFlush()
-    // calls), so the limit is larger than the expected number of images.  The
-    // InterleavedAttributeDataBenchmark perf test for example issues a large number of flushes.
-    ASSERT(mInFlightCommands.size() <= kInFlightCommandsLimit);
-
     ANGLE_TRY(checkCompletedCommands(context));
 
+    // CPU should be throttled to avoid mInFlightCommands from growing too fast. Important for
+    // off-screen scenarios.
+    while (mInFlightCommands.size() > kInFlightCommandsLimit)
+    {
+        ANGLE_TRY(finishToSerial(context, mInFlightCommands[0].serial,
+                                 renderer->getMaxFenceWaitTimeNs()));
+    }
+
     return angle::Result::Continue;
 }