Don't pass preferred size from thumbnail capturing

Thumbnail capturing is a hidden capturer which should not have
side-effect. It should not prefer the capture size.

This CL is self-contained and tests by itself with the DCHECK in
WebContentsImpl::IncrementCapturerCount(). I've verified that all
callers now meet the requirement.

Also verified programatically that the thumbnail capturing won't cause
side effect of page rendering.

Bug: 1112607
Change-Id: I9f9618e75568cec02d48ddb4b73c43b6233b7fa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380884Reviewed-by: Dana Fried <dfried@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803645}

chrome/browser/ui/thumbnails/thumbnail_tab_helper.cc

@@ -64,10 +64,7 @@ class ScopedThumbnailCapture {
       : web_contents_observer_(web_contents_observer) {
     auto* const contents = web_contents_observer->web_contents();
     if (contents) {
-      contents->IncrementCapturerCount(
-          gfx::ScaleToFlooredSize(GetMinimumThumbnailSize(),
-                                  kMinThumbnailScaleFactor),
-          /* stay_hidden */ true);
+      contents->IncrementCapturerCount(gfx::Size(), /* stay_hidden */ true);
       captured_ = true;
     }
   }

content/browser/web_contents/web_contents_impl.cc

@@ -1700,10 +1700,14 @@ void WebContentsImpl::SetWasDiscarded(bool was_discarded) {
 void WebContentsImpl::IncrementCapturerCount(const gfx::Size& capture_size,
                                              bool stay_hidden) {
   DCHECK(!is_being_destroyed_);
-  if (stay_hidden)
+  if (stay_hidden) {
+    // A hidden capture should not have side effect on the web contents, so it
+    // should not pass a non-empty |capture_size| which will cause side effect.
+    DCHECK(capture_size.IsEmpty());
     ++hidden_capturer_count_;
-  else
+  } else {
     ++visible_capturer_count_;
+  }
 
   // Note: This provides a hint to upstream code to size the views optimally
   // for quality (e.g., to avoid scaling).