Namespace Log.h/cpp in dawn::

The LogMessage::LogMessage constructor was redefining a symbol that
exists in re2 inside of Chromium. So we namespace Log inside dawn:: to
avoid the conflict.

BUG=dawn:302

Change-Id: Ida349208e2c6fe9ac032e1bd8cd442dff0b3f6bc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14320
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/tests/perf_tests/DawnPerfTest.cpp

@@ -113,7 +113,7 @@ DawnPerfTestEnvironment::DawnPerfTestEnvironment(int argc, char** argv)
         }
 
         if (strcmp("-h", argv[i]) == 0 || strcmp("--help", argv[i]) == 0) {
-            InfoLog()
+            dawn::InfoLog()
                 << "Additional flags:"
                 << " [--calibration] [--override-steps=x] [--enable-tracing] [--trace-file=file]\n"
                 << "  --calibration: Only run calibration. Calibration allows the perf test"