Simplify content_layer attachment and viewporting logic.

chrome/browser/android/compositor/layer/thumbnail_layer.cc

Index: chrome/browser/android/compositor/layer/thumbnail_layer.cc
diff --git a/chrome/browser/android/compositor/layer/thumbnail_layer.cc b/chrome/browser/android/compositor/layer/thumbnail_layer.cc
index 169f26c6708311c115d07e5b5ddadc451de65780..007a17b86a830b473fd75e8800cb6931a29e21c4 100644
--- a/chrome/browser/android/compositor/layer/thumbnail_layer.cc
+++ b/chrome/browser/android/compositor/layer/thumbnail_layer.cc
@@ -23,6 +23,8 @@ void ThumbnailLayer::SetThumbnail(Thumbnail* thumbnail) {
 
 void ThumbnailLayer::Clip(const gfx::Rect& clipping) {
   last_clipping_ = clipping;
+  clipped_ = true;
+
   gfx::Size clipped_content = gfx::Size(content_size_.width() - clipping.x(),
                                         content_size_.height() - clipping.y());
   clipped_content.SetToMin(clipping.size());
@@ -36,6 +38,11 @@ void ThumbnailLayer::Clip(const gfx::Rect& clipping) {
           (clipping.y() + clipped_content.height()) / resource_size_.height()));
 }
 
+void ThumbnailLayer::ClearClip() {
+  layer_->SetUV(gfx::PointF(0.f, 0.f), gfx::PointF(1.f, 1.f));
+  clipped_ = false;
+}
+
 void ThumbnailLayer::AddSelfToParentOrReplaceAt(scoped_refptr<cc::Layer> parent,
                                                 size_t index) {
   if (index >= parent->children().size())
@@ -60,7 +67,8 @@ void ThumbnailLayer::UpdateSizes(const gfx::SizeF& content_size,
   if (content_size != content_size_ || resource_size != resource_size_) {
     content_size_ = content_size;
     resource_size_ = resource_size;
-    Clip(last_clipping_);
+    if (clipped_)
+      Clip(last_clipping_);
   }
 }