Make IDBTransaction ctor take ScriptState instead of ExecutionContext

Make IDBTransaction ctor take ScriptState instead of ExecutionContext

This looks like a no-op, but forthcoming changes to
IDBPendingTransactionMonitor require this and it makes those patches
much easier to understand.

BUG=390704, 411532
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=181782

[jsbell, 2014-09-10 21:41:37]

jsbell@chromium.org changed reviewers:
+ adamk@chromium.org, haraken@chromium.org, tasak@chromium.org

[jsbell, 2014-09-10 21:41:38]

Prospective chain of patches is:

1. https://codereview.chromium.org/555633002 (this patch) -
ExecutionContext->ScriptState in IDBTransaction ctor
2. https://codereview.chromium.org/429453010 - drop ExecutionContext dependency
from V8RecursionScope
3. https://codereview.chromium.org/555163005 - wrap Microtask execution with
V8RecursionScope

haraken@ - please take a look?

Biggest questions/issues are in #2.

[haraken, 2014-09-11 01:02:45]

This CL LGTM.

We always prefer ScriptState than ExecutionContext (except that retrieving
ScriptState is a bit slower than retrieving ExecutionContext; but this won't
become an issue except for extremely performance-sensitive cases).

[haraken, 2014-09-11 01:02:56]

The CQ bit was checked by haraken@chromium.org

[commit-bot: I haz the power, 2014-09-11 01:03:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/555633002/1

[commit-bot: I haz the power, 2014-09-11 02:29:29]

Committed patchset #1 (id:1) as 181782