Avoid data race when writing Shell::options.script_executed.

Avoid data race when writing Shell::options.script_executed.

The race occurred when Workers were used. Since Workers call
Shell::ExecuteString from a different thread, TSAN (correctly) flags
this as a racy write. Solution would be to either synchronize the writes,
or to 'lift' the write higher up in the call stack and only write the flag
from the main thread. This implements this latter solution.

These methods call Shell::ExecuteString, but do *not* set script_executed:
- ExecuteInThread: Can only occur is JS has already been executed.
- Shell::Load: Callback for JS; so JS has already been executed when
               we get there.
- Shell::RunShell: Interactive shell. We no longer need script_executed once
                   we're here.

BUG=v8:4330
LOG=N

Committed: https://crrev.com/e045b78d8ed1d0450f70baa44d16eda78f680188
Cr-Commit-Position: refs/heads/master@{#30003}

[vogelheim, 2015-08-04 14:07:55]

vogelheim@chromium.org changed reviewers:
+ yangguo@chromium.org

[Yang, 2015-08-04 14:10:12]

On 2015/08/04 14:08:49, vogelheim wrote:

lgtm.

[vogelheim, 2015-08-04 14:25:06]

The CQ bit was checked by vogelheim@chromium.org

[commit-bot: I haz the power, 2015-08-04 14:25:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1258303004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1258303004/1

[commit-bot: I haz the power, 2015-08-04 14:28:07]

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

[commit-bot: I haz the power, 2015-08-04 14:28:07]

Try jobs failed on following builders:
  v8_presubmit on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_presubmit/builds/4678)

[vogelheim, 2015-08-04 14:30:05]

The CQ bit was checked by vogelheim@chromium.org

[commit-bot: I haz the power, 2015-08-04 14:30:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1258303004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1258303004/1

[commit-bot: I haz the power, 2015-08-04 14:31:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-04 14:32:39]

Patchset 1 (id:??) landed as
https://crrev.com/e045b78d8ed1d0450f70baa44d16eda78f680188
Cr-Commit-Position: refs/heads/master@{#30003}