build/config: use "trim string" instead of "value" for clang revision

build/config: use "trim string" instead of "value" for clang revision

Clang revisions aren't always GN literals; e.g., right now it's
"233105-1", but GN's "value" conversion parses as just "233105",
silently ignoring the trailing "-1".

See also discussion on gn-dev:
https://groups.google.com/a/chromium.org/d/msg/gn-dev/hHRC1HuWBo8/drOd5dTvO5oJ

Committed: https://crrev.com/985f4d610a682fb4059d0360fc4de9da45bf34d8
Cr-Commit-Position: refs/heads/master@{#322874}

[mdempsky, 2015-03-30 20:20:20]

mdempsky@chromium.org changed reviewers:
+ scottmg@chromium.org, thakis@chromium.org

[scottmg, 2015-03-30 20:22:11]

I might have actually expected 233104. :) lgtm

[mdempsky, 2015-03-30 20:59:59]

The CQ bit was checked by mdempsky@chromium.org

[commit-bot: I haz the power, 2015-03-30 21:00:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1049453002/1

[Nico, 2015-03-30 21:02:01]

lgtm

Thanks much :-)

[commit-bot: I haz the power, 2015-03-30 21:37:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-30 21:38:42]

Patchset 1 (id:??) landed as
https://crrev.com/985f4d610a682fb4059d0360fc4de9da45bf34d8
Cr-Commit-Position: refs/heads/master@{#322874}