Add convenience method for converting v8::PersistentBase to v8::Local

Add convenience method for converting v8::PersistentBase to v8::Local

The CL addes convenienve method that allows to write code like the following
v8::Local<v8::Object> local = v8::Local<v8::Object>::New(global, isolate);
in a more readable way:
v8::Local<v8::Object> local = global.Get(isolate);

There is already v8::Eternal::Get that does similar thing.

BUG=None
LOG=Y

Committed: https://crrev.com/e5a77abcf8c651a1abd3949540ce6c9100ced4ef
Cr-Commit-Position: refs/heads/master@{#29616}

[yurys, 2015-07-13 13:22:43]

yurys@chromium.org changed reviewers:
+ jochen@chromium.org

[yurys, 2015-07-13 13:24:36]

The CQ bit was checked by yurys@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-13 13:24:45]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1237603003/1

[jochen (gone - plz use gerrit), 2015-07-13 13:24:46]

lgtm

[yurys, 2015-07-13 13:25:03]

The CQ bit was unchecked by yurys@chromium.org

[yurys, 2015-07-13 13:25:09]

The CQ bit was checked by yurys@chromium.org

[commit-bot: I haz the power, 2015-07-13 13:25:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1237603003/1

[commit-bot: I haz the power, 2015-07-13 13:30:54]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-13 13:30:55]

Try jobs failed on following builders:
  v8_linux64_avx2_rel on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_linux64_avx2_rel/builds/...)

[yurys, 2015-07-13 14:13:55]

The CQ bit was checked by yurys@chromium.org

[yurys, 2015-07-13 14:13:55]

The patchset sent to the CQ was uploaded after l-g-t-m from jochen@chromium.org
Link to the patchset: https://codereview.chromium.org/1237603003/#ps20001
(title: "Fixed test compilation")

[commit-bot: I haz the power, 2015-07-13 14:14:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1237603003/20001

[commit-bot: I haz the power, 2015-07-13 15:02:25]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-07-13 15:02:39]

Patchset 2 (id:??) landed as
https://crrev.com/e5a77abcf8c651a1abd3949540ce6c9100ced4ef
Cr-Commit-Position: refs/heads/master@{#29616}