Default to 1 when --force-device-scale-factor has an invalid value.

Default to 1 when --force-device-scale-factor has an invalid value.

Currently, the scaling factor may end up as 0, causing a divide by 0.
Also add warnings to base::StringToFoo().

BUG=517944
Committed: https://crrev.com/62d4c69ec6bfaff365d9b240c1519802ddd245ab
Cr-Commit-Position: refs/heads/master@{#342505}

[Lei Zhang, 2015-08-07 22:15:43]

thestig@chromium.org changed reviewers:
+ msw@chromium.org

[Lei Zhang, 2015-08-07 22:15:43]

msw: I'd ask oshima, but he's OOO.

[Lei Zhang, 2015-08-07 22:25:01]

And now with unit test.

[oshima, 2015-08-08 00:12:25]

oshima@chromium.org changed reviewers:
+ oshima@chromium.org

[oshima, 2015-08-08 00:12:25]

lgtm

[Lei Zhang, 2015-08-08 00:19:25]

The CQ bit was checked by thestig@chromium.org

[commit-bot: I haz the power, 2015-08-08 00:20:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1278173006/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1278173006/20001

[commit-bot: I haz the power, 2015-08-08 02:27:13]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-08 02:27:44]

Patchset 2 (id:??) landed as
https://crrev.com/62d4c69ec6bfaff365d9b240c1519802ddd245ab
Cr-Commit-Position: refs/heads/master@{#342505}