[CSS Grid Layout] Improve verbosity of test results

CSS Grid Layout testing code was recently refactored to remove a bunch
of huge copy&pastes. The problem with that is that it replaced part of
the tests self-descriptive expected results by "dull" lines with almost
no meaning. This change restores the previous output while keeping the
nice refactorings.

Review URL: https://chromiumcodereview.appspot.com/22854033

git-svn-id: svn://svn.chromium.org/blink/trunk@157146 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Change Canvas2DLayerBridge to stay alive until last mailbox is returned.

This change modifies the destruction mechanism for Canvas2DLayerBridge.
When the owning ImageBuffer object is destroyed, the Canvas2DLayerBridge
will no longer be destroyed if it has unreleased texture mailboxes.
Instead, the object is leaked and becomes responsible for self-destructing
as soon as its outstanding texture mailboxes are returned by the compositor.

BUG=274113
TEST=leak and use after free verified by valgrind and asan layout test runs. Proper tear-down inspected and validated manually using debugger.
R=senorblanco@chromium.org,piman@chromium.org,danakj@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22929012

git-svn-id: svn://svn.chromium.org/blink/trunk@157147 bbb929c8-8fbe-4397-9dbb-9b2b20218538