Do not hold references to ExecutionContext from DatabaseBase.

We don't need to hold RefPtr<ExecutionContext> in DatabaseBase because Database
and DatabaseSync, which are derived from DatabaseBase, are derived from
DatabaseBackendBase too, and we can obtain ExecutionContext from DatabaseContext
associated to DatabaseBackendBase.

DatabaseBase::logErrorMessage is moved to DatabaseBackendBase because we can't
access ExecutionContext in DatabaseBase any longer.

BUG=340522
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/181023002

git-svn-id: svn://svn.chromium.org/blink/trunk@167875 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Allow arbitrary values for [SetWrapperReferenceFrom]

Per Adam Klein's comment at:
https://codereview.chromium.org/161083002/#msg4
this CL removes the explicit list of allowed values from
[SetWrapperReferenceFrom], since it's just an arbitrary function name.

Per haraken's comment at:
https://codereview.chromium.org/161083002/#msg5
this is ok, so TBR'ing.

TBR=haraken
BUG=345503

Review URL: https://codereview.chromium.org/181133002

git-svn-id: svn://svn.chromium.org/blink/trunk@167876 bbb929c8-8fbe-4397-9dbb-9b2b20218538