Don't purge resources for trivial GrContext flushes

tests/ResourceCacheTest.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 // Include here to ensure SK_SUPPORT_GPU is set correctly before it is examined.
 #include "SkTypes.h"
 
@@ skipping 1177 matching lines @@
         for (int i = 0; i < kFlushCount; ++i) {
             REPORTER_ASSERT(reporter, kFlushCount >> 1 == cache->getResourceCount());
             cache->notifyFlushOccurred();
         }
         REPORTER_ASSERT(reporter, 0 == cache->getResourceCount());
 
         cache->purgeAllUnlocked();
     }
 
     REPORTER_ASSERT(reporter, 0 == cache->getResourceCount());
+
+    // Verify that calling flush() on a GrContext with nothing to do will not trigger resource
+    // eviction.
+    context->flush();
+    for (int i = 0; i < 10; ++i) {
+        TestResource* r = new TestResource(context->getGpu());
+        GrUniqueKey k;
+        make_unique_key<1>(&k, i);
+        r->resourcePriv().setUniqueKey(k);
+        r->unref();
+    }
+    REPORTER_ASSERT(reporter, 10 == cache->getResourceCount());
+    for (int i = 0; i < 10 * kFlushCount; ++i) {
+        context->flush();
+    }
+    REPORTER_ASSERT(reporter, 10 == cache->getResourceCount());
 }
 
 static void test_large_resource_count(skiatest::Reporter* reporter) {
     // Set the cache size to double the resource count because we're going to create 2x that number
     // resources, using two different key domains. Add a little slop to the bytes because we resize
     // down to 1 byte after creating the resource.
     static const int kResourceCnt = 2000;
 
     Mock mock(2 * kResourceCnt, 2 * kResourceCnt + 1000);
     GrContext* context = mock.context();
@@ skipping 104 matching lines @@
     test_cache_chained_purge(reporter);
     test_resource_size_changed(reporter);
     test_timestamp_wrap(reporter);
     test_flush(reporter);
     test_large_resource_count(reporter);
     test_custom_data(reporter);
     test_abandoned(reporter);
 }
 
 #endif