Add Log.h to replace all uses of iostream

On Android iostream doesn't appear in logcat, the system log that's
often used for printf debugging. Introduce Chromium/ANGLE like logging
that looks like the following:

  InfoLog() << stuff << stuff;

This makes sure the message is put in logcat on Android and removes
static initializers from <iostream>

BUG=dawn:286

Change-Id: Ie0d018f49bcac1a7b740739a6e59d45ae6728638
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14102
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: David Turner <digit@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>

src/common/Assert.cpp

@@ -13,14 +13,13 @@
 // limitations under the License.
 
 #include "common/Assert.h"
-
-#include <iostream>
+#include "common/Log.h"
 
 void HandleAssertionFailure(const char* file,
                             const char* function,
                             int line,
                             const char* condition) {
-    std::cerr << "Assertion failure at " << file << ":" << line << " (" << function
-              << "): " << condition << std::endl;
+    ErrorLog() << "Assertion failure at " << file << ":" << line << " (" << function
+               << "): " << condition;
     DAWN_BREAKPOINT();
 }