Add another null check for the Vk NexusPlayer bot

That bot has flaky failures creating contexts, and it consistently fails
this test. It probably can't create two contexts at the same time.

BUG=skia:

Change-Id: If6363bff22882378f51281f1cbdac8b3ef510af2
Reviewed-on: https://skia-review.googlesource.com/9067
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

tests/GrContextFactoryTest.cpp

@@ -117,6 +117,10 @@ DEF_GPUTEST(GrContextFactory_sharedContexts, reporter, /*factory*/) {
 
         // Create a new base context
         ContextInfo info3 = testFactory.getContextInfo(ctxType);
+        if (!info3.grContext()) {
+            // Vulkan NexusPlayer bot fails here. Sigh.
+            continue;
+        }
 
         // Creating a context in a share group may fail, but should never crash.
         ContextInfo info4 = testFactory.getSharedContextInfo(info3.grContext());