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

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

crrev.com/1016603004 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/bb9c774c164c3552b67b4f1b60809852a44f2ecd
Cr-Commit-Position: refs/heads/master@{#28858}

[vogelheim, 2015-06-09 09:53:40]

The CQ bit was checked by vogelheim@chromium.org

[commit-bot: I haz the power, 2015-06-09 09:53:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1041683002/40001

[commit-bot: I haz the power, 2015-06-09 09:53:50]

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

[commit-bot: I haz the power, 2015-06-09 09:53:50]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[vogelheim, 2015-06-09 09:56:39]

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

[vogelheim, 2015-06-09 09:56:40]

Commit-bot is right: I mentioned this in person, but forgot about actual review.
:-|  Care to have a look?

[jochen (gone - plz use gerrit), 2015-06-09 09:57:59]

lgtm

[vogelheim, 2015-06-09 09:58:12]

The CQ bit was checked by vogelheim@chromium.org

[commit-bot: I haz the power, 2015-06-09 09:58:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1041683002/40001

[commit-bot: I haz the power, 2015-06-09 09:59:27]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-06-09 09:59:37]

Patchset 3 (id:??) landed as
https://crrev.com/bb9c774c164c3552b67b4f1b60809852a44f2ecd
Cr-Commit-Position: refs/heads/master@{#28858}

[vogelheim, 2015-06-11 08:31:54]

A revert of this CL (patchset #3 id:40001) has been created in
https://codereview.chromium.org/1177883003/ by vogelheim@chromium.org.

The reason for reverting is: Revert, due to problems with stale build artefacts
uncovered by this.

Details: https://code.google.com/p/v8/issues/detail?id=4171 .