Re-Re-land: Enable external startup by default on Linux.

Re-Re-land: Enable external startup by default on Linux.

crrev.com/1177163002 should have fixed the issues that broke this last time.

Notes:
- The current functionality is meant to stay. If you prefer the old way,
  just set v8_use_external_startup_data=0.
- You might notice this if you call d8 via PATH. Either explicitly tell d8
  where your external snapshot/built-ins are, or change the build options
  as above.
- This follows Chromium practice, that mostly uses this feature these days.
- Other platforms to follow later.

BUG=

Committed: https://crrev.com/17c1cf28657cbb6163cdf488eabeec1e620ceff7
Cr-Commit-Position: refs/heads/master@{#29050}

[vogelheim, 2015-06-16 07:41:43]

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

[vogelheim, 2015-06-16 07:41:44]

Let's try this again. The fix for the make generator is in
(https://codereview.chromium.org/1187073008).

Last time, we had several funny build errors within the first 24h, so I'd take
that as my probabtion period for this CL.. :-)

[Michael Achenbach, 2015-06-16 11:14:28]

lgtm - just land at any time - maybe you can make a test CL that changes some JS
file for the snapshot. Or a test revert of one of the changes that had the
problem (just on the tryserver).

[vogelheim, 2015-06-16 11:25:44]

The CQ bit was checked by vogelheim@chromium.org

[commit-bot: I haz the power, 2015-06-16 11:25:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1189563004/1

[commit-bot: I haz the power, 2015-06-16 11:26:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-16 11:27:12]

Patchset 1 (id:??) landed as
https://crrev.com/17c1cf28657cbb6163cdf488eabeec1e620ceff7
Cr-Commit-Position: refs/heads/master@{#29050}