[BlinkGenPropertyTrees] Print logging per-line

This patch prints the property tree logging by printing each line
separately.

This prevents truncation of the logging output on Android, where:

  LOG << ...long multiline string...

Will be truncated after some finite number of characters. For long
outputs such as the property trees we hit this quite easily.

Bug: 854200
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I7e70a5e66d8248b2faa478738db94c980c28e3f7
Reviewed-on: https://chromium-review.googlesource.com/1173156
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582744}

cc/trees/layer_tree_host.cc

@@ -791,7 +791,12 @@ bool LayerTreeHost::DoUpdateLayers(Layer* root_layer) {
       base::JSONWriter::WriteWithOptions(
           *property_trees->AsTracedValue()->ToBaseValue(),
           base::JSONWriter::OPTIONS_PRETTY_PRINT, &out);
-      VLOG(3) << out;
+      std::stringstream ss(out);
+      while (!ss.eof()) {
+        std::string line;
+        std::getline(ss, line);
+        VLOG(3) << line;
+      }
     }
   }