[turbofan] Preserve cached code across GCs.

[turbofan] Preserve cached code across GCs.

This preserves the context-independent entry in an optimized code map
across GCs when the code is considered young (i.e. less than 3 ages).
Note that any context-dependent entry for the same code will still be
flushed immediately when the respective context dies, hence context
lifetime is not increased.

R=hpayer@chromium.org

Committed: https://crrev.com/2c07b0d045829da2e61e7c94bdcba89581e4866b
Cr-Commit-Position: refs/heads/master@{#29790}

[Michael Starzinger, 2015-07-22 11:59:22]

https://codereview.chromium.org/1252463002/diff/20001/src/flag-definitions.h
File src/flag-definitions.h (right):

https://codereview.chromium.org/1252463002/diff/20001/src/flag-definitions.h#...
src/flag-definitions.h:434: DEFINE_BOOL(turbo_preserve_shared_code, false, "keep
context-independent code")
Note that this is still off, I want to enable it in a sparate CL to allow for
easy reverts in case of regressions.

[Hannes Payer (out of office), 2015-07-22 12:46:46]

lgtm

[Michael Starzinger, 2015-07-22 14:02:11]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-07-22 14:02:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1252463002/20001

[commit-bot: I haz the power, 2015-07-22 14:04:09]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-07-22 14:04:14]

Patchset 2 (id:??) landed as
https://crrev.com/2c07b0d045829da2e61e7c94bdcba89581e4866b
Cr-Commit-Position: refs/heads/master@{#29790}