Add new isolation mode "prepare".

Add new isolation mode "prepare".

In that mode isolate_driver.py will collect all arguments needed for
isolate.py invocation into *.isolated.gen.json file, but won't actually
run the isolation itself.

It's a feature of isolate_driver.py. Isolate client itself knows nothing about
this mode.

BUG=389227
R=maruel@chromium.org

Committed: https://crrev.com/8556ebdc21ada4753aa6798a240c83101f212dcf
Cr-Commit-Position: refs/heads/master@{#293885}

[Vadim Sh., 2014-09-08 21:45:00]

ptal

generates a file that looks like this:
{
  "args": [
    "--isolated", 
    "../out/Debug/base_unittests.isolated", 
    "--isolate", 
    "base_unittests.isolate", 
    "--path-variable", 
    "DEPTH", 
    "..", 
    "--path-variable", 
    "PRODUCT_DIR", 
    "../out/Debug ", 
    "--extra-variable", 
    "version_full=39.0.2147.0", 
    "--config-variable", 
    "OS=linux", 
    "--config-variable", 
    "CONFIGURATION_NAME=Debug", 
    "--config-variable", 
    "chromeos=0", 
    "--config-variable", 
    "component=static_library", 
    "--config-variable", 
    "fastbuild=0", 
    "--config-variable", 
    "internal_gles2_conform_tests=0", 
    "--config-variable", 
    "icu_use_data_file_flag=1", 
    "--config-variable", 
    "libpeer_target_type=static_library", 
    "--config-variable", 
    "use_openssl=0", 
    "--config-variable", 
    "target_arch=x64", 
    "--ignore_broken_items"
  ], 
  "dir": "/home/vadimsh/Desktop/all-chromium/src/base", 
  "version": "1"
}

[Vadim Sh., 2014-09-08 22:19:02]

actually wait a bit, I'm writing isolate.py side of this. And it might be easier
to "parse" arguments right here and pass them to isolate.py in more structured
form.

[M-A Ruel, 2014-09-08 22:40:29]

lgtm

https://codereview.chromium.org/555553002/diff/20001/tools/isolate_driver.py
File tools/isolate_driver.py (right):

https://codereview.chromium.org/555553002/diff/20001/tools/isolate_driver.py#...
tools/isolate_driver.py:257: with open(output, 'wt') as f:
Why not 'wb'? I hate CR with passion. :)

[Vadim Sh., 2014-09-08 23:57:19]

I decided to commit this as is (i.e. JSON contains command line).

https://codereview.chromium.org/555553002/diff/20001/tools/isolate_driver.py
File tools/isolate_driver.py (right):

https://codereview.chromium.org/555553002/diff/20001/tools/isolate_driver.py#...
tools/isolate_driver.py:257: with open(output, 'wt') as f:
On 2014/09/08 22:40:28, M-A Ruel wrote:
> Why not 'wb'? I hate CR with passion. :)

Done.

[Vadim Sh., 2014-09-08 23:57:57]

The CQ bit was checked by vadimsh@chromium.org

[commit-bot: I haz the power, 2014-09-09 00:28:48]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/vadimsh@chromium.org/555553002/40001

[commit-bot: I haz the power, 2014-09-09 07:25:08]

Committed patchset #3 (id:40001) as bc3cb6bd399df5e55629e3d656aadafd0b6fc911

[commit-bot: I haz the power, 2014-09-10 03:51:39]

Patchset 3 (id:??) landed as
https://crrev.com/8556ebdc21ada4753aa6798a240c83101f212dcf
Cr-Commit-Position: refs/heads/master@{#293885}