Invalidate paint when a squashed layer becomes individually composited *before* clearing out the groupedMapping.

Previously we were clearing the groupedMapping first, which
means that the invalidation happened with respect to the
graphics layer of the nearest compositing ancestor of the
layer, which is incorrect. It needs to be the squashing
layer.

BUG=398833

Review URL: https://codereview.chromium.org/442083002

git-svn-id: svn://svn.chromium.org/blink/trunk@179581 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Save File::userVisibility() in structured cloning.

File instances have a userVisibility() getter that is used to
distinguish between files selected by the user in a file picker (e.g.,
the one invoked by <input type="file">) and files created by JavaScript
(using the File constructor or the Filesystem API). The distinction is
important, for example, when deciding if an <input type="file"> with a
currently selected file should launch a file picker focused on the
file's directory. Details at http://crbug.com/360308

This change updates the (non-indexed Blobs version of the) structured
clone code to preserve a File's userVisibility, so we still make the
right decisions when an <input type="file"> is assigned a FileList
received via postMessage.

BUG=398366

Review URL: https://codereview.chromium.org/426393005

git-svn-id: svn://svn.chromium.org/blink/trunk@179582 bbb929c8-8fbe-4397-9dbb-9b2b20218538