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/common/Assert.cpp

@@ -19,7 +19,7 @@ void HandleAssertionFailure(const char* file,
                             const char* function,
                             int line,
                             const char* condition) {
-    ErrorLog() << "Assertion failure at " << file << ":" << line << " (" << function
-               << "): " << condition;
+    dawn::ErrorLog() << "Assertion failure at " << file << ":" << line << " (" << function
+                     << "): " << condition;
     DAWN_BREAKPOINT();
 }