Fix test failures of IDBTransactionTest.EnsureLifetime with ENABLE(OILPAN)

This is a regression caused by r174937.
After r174937, ExecutionContexts created for IDBRequestTest and IDBTransactionTest
outlive the tests (because the lifetime of the ExecutionContexts is bound to a ScriptState
associated with the tests). This is wrong, and we should clear the ExecutionContexts
when shutting down the tests.

BUG=379616

Review URL: https://codereview.chromium.org/307933011

git-svn-id: svn://svn.chromium.org/blink/trunk@175259 bbb929c8-8fbe-4397-9dbb-9b2b20218538