Remove SetIsDrawable() from WebContentLayerImpl constructor.

Each caller of CreateContentLayer() calls SetDrawsContent() explicitly
already, which sets this same bool, so there is no need to call it in
the constructor or add any other calls.

R=pdr@chromium.org

Bug: 838693
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iec4c50d3334c733ac62f6756a3e40f392e1dd983
Reviewed-on: https://chromium-review.googlesource.com/1044897Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556249}

cc/blink/web_content_layer_impl.cc

@@ -47,7 +47,6 @@ PaintingControlToWeb(
 WebContentLayerImpl::WebContentLayerImpl(blink::WebContentLayerClient* client)
     : client_(client) {
   layer_ = std::make_unique<WebLayerImpl>(PictureLayer::Create(this));
-  layer_->layer()->SetIsDrawable(true);
 }
 
 WebContentLayerImpl::~WebContentLayerImpl() {