Prefix glmark2 perf results with glmark2_

Bug: angleproject:3125
Change-Id: Ifa45d489cb082355a23f33f05f805e5f6162ac0c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1529232
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

src/tests/perf_tests/glmark2Benchmark.cpp

@@ -244,7 +244,9 @@ class GLMark2Benchmark : public testing::TestWithParam<GLMark2BenchmarkTestParam
 
             if (!completeRun)
             {
-                perf_test::PrintResult(benchmarkName, '_' + mBackend, "fps", fps, "", true);
+                const std::string kBenchmarkPrefix = "glmark2_";
+                perf_test::PrintResult(kBenchmarkPrefix + benchmarkName, '_' + mBackend, "fps", fps,
+                                       "", true);
             }
         }