Prevent use of NULL IndexedDB backend database.

When an IndexedDB connection (IDBDatabase) is closed it closes it's backend
db (WebIDBDatabase) and set's it's ptr to NULL. If a transaction is used
after that, it may attempt to use that NULL backend db ptr. This change adds
a NULL ptr check before use to avoid a crash.

Speculative fix for crbug.com/361278

BUG=361278

Review URL: https://codereview.chromium.org/230503006

git-svn-id: svn://svn.chromium.org/blink/trunk@171174 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove the compositing/iframes/iframe-src-change.html test.

This test checks that the return value of layerTreeAsText() is not
empty, but it is never empty when we are in force-compositing-mode, so
the test is not testing anything useful anymore.

R=enne@chromium.org
BUG=361729

Review URL: https://codereview.chromium.org/231643003

git-svn-id: svn://svn.chromium.org/blink/trunk@171175 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix checkdeps in third_party/WebKit.

Running checkdeps normally skips third_party subdirectories. However when uploading from third_party/WebKit, the presubmit checks run checkdeps on new include lines. I can't find a way to make checkdeps not ignore third_party/WebKit when checking the root src directory, so until we merge repos this might regress.

Review URL: https://codereview.chromium.org/231303002

git-svn-id: svn://svn.chromium.org/blink/trunk@171176 bbb929c8-8fbe-4397-9dbb-9b2b20218538