Fix use of 'sysroot' variable in harfbuzz.gyp

Fix use of 'sysroot' variable in harfbuzz.gyp

The sysroot variable is set conditionally in common.gypi
but not at the nesting level required for other conditional
to depend on it like the one in hardbuzz does.  This means
that sysroot always appeared to be empty to harfbuzz.gyp,
and hence the special handling for ARM linux was required.
This change moves the setting of 'sysroot' up one level of
nesting and defines 'pkg-config' in common.gypi so it can
be shared.

This is the third attempt at a landing. First attempt was
reverted: https://codereview.chromium.org/1151753002

This is a re-land of the second attempt which was
reverted speculatively (wrongly as it turned out):
https://codereview.chromium.org/1158283003

TBR=cpu

Committed: https://crrev.com/dea1c634b5328b1e171ba22698fb7027f338453f
Cr-Commit-Position: refs/heads/master@{#332982}

[Sam Clegg, 2015-06-04 22:26:06]

sbc@chromium.org changed reviewers:
+ cpu@chromium.org

[Sam Clegg, 2015-06-05 01:07:28]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-06-05 01:08:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1146303007/1

[commit-bot: I haz the power, 2015-06-05 01:13:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-05 01:14:46]

Patchset 1 (id:??) landed as
https://crrev.com/dea1c634b5328b1e171ba22698fb7027f338453f
Cr-Commit-Position: refs/heads/master@{#332982}