Add ScriptValue.v8ValueUnsafe() to access the V8 value without cross-context access check.

In DevTools instrumentation we may need to access an async stack trace object
wrapped into a ScriptValue from a different world. We would unwrap it to a
native object only to get it's String and number members, and not passing the
object to user's scripts.

BUG=396916
R=yurys

Review URL: https://codereview.chromium.org/418823002

git-svn-id: svn://svn.chromium.org/blink/trunk@178944 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add explicit destructor to DatabaseBackend.h.

This class was missed in https://codereview.chromium.org/412483007/, see that
CL's description for the motivation.

I filed http://llvm.org/20452 about the diagnostic for this not being as clear
as it could be.

BUG=82385
R=rnk@chromium.org
TBR=michaeln

Review URL: https://codereview.chromium.org/419853002

git-svn-id: svn://svn.chromium.org/blink/trunk@178945 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add a frame sequence number to HistoryItem

This will be used by the history system in content/renderer/ to disambiguate
frames during history navigation.

The current system uses two factors to try to find frames for navigation:
frame routing ids, and frame unique names. Routing ids can't be stored for
later use, because they might get saved to disk and collide with routing ids
in a future renderer process. Unique names are more reliable...unless a
website modifies window.name, at which point we try to do lookups based on a
stale name and can't find the right frame.

Instead, we introduce a frame sequence number. Like the existing item and
document sequence numbers, these can be saved to disk with a low chance of
collisions. These can then be reliably used during the current session to
match frames for back/forward navigations.

BUG=393227,395097

Review URL: https://codereview.chromium.org/418103003

git-svn-id: svn://svn.chromium.org/blink/trunk@178947 bbb929c8-8fbe-4397-9dbb-9b2b20218538