Fix VKEY determination for non-US layouts under Ozone.

Fix VKEY determination for non-US layouts under Ozone.

We attempt to match Windows behaviour for the value of VKEY on
a keystroke. Some XKB layouts generate characters for AltGr+key
combinations that are not present in the reference (Windows-based)
sources, which led to mismtaches in the mappings, and incorrect VKEY
values for those keys. To avoid this, we match on any AltGr+key result
in cases the reference has none.

The corresponding test is revised to use a valid dummy character
for those cases, rather than 'NoSymbol'.

BUG=480529
Committed: https://crrev.com/e63d52e7a86a8cb5f6703b1ae98f44be1a85de35
Cr-Commit-Position: refs/heads/master@{#326695}

[kpschoedel, 2015-04-23 23:25:42]

The CQ bit was checked by kpschoedel@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-04-23 23:26:06]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1100393005/1

[kpschoedel, 2015-04-23 23:27:54]

kpschoedel@chromium.org changed reviewers:
+ spang@chromium.org

[spang, 2015-04-23 23:37:56]

lgtm

[commit-bot: I haz the power, 2015-04-23 23:48:12]

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

[commit-bot: I haz the power, 2015-04-23 23:48:13]

Dry run: This issue passed the CQ dry run.

[kpschoedel, 2015-04-23 23:57:31]

The CQ bit was checked by kpschoedel@chromium.org

[commit-bot: I haz the power, 2015-04-23 23:58:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1100393005/1

[commit-bot: I haz the power, 2015-04-23 23:59:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-24 00:00:40]

Patchset 1 (id:??) landed as
https://crrev.com/e63d52e7a86a8cb5f6703b1ae98f44be1a85de35
Cr-Commit-Position: refs/heads/master@{#326695}