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.

Second attempt at landing this. First attempt was here:
https://codereview.chromium.org/1151753002

TBR=cpu

Committed: https://crrev.com/fa36d7041817ddd299c89ca5ecef432c9210fda9
Cr-Commit-Position: refs/heads/master@{#332311}

[Sam Clegg, 2015-05-29 21:42:54]

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

[Sam Clegg, 2015-05-29 21:43:30]

This is a re-land of a previous CL (the chromite bug it exposed is now fixed)..
so I guess I'll TBR this.

[Sam Clegg, 2015-05-29 21:43:37]

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

[Dirk Pranke, 2015-05-29 21:50:20]

rs lgtm.

[commit-bot: I haz the power, 2015-05-29 22:02:06]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1158283003/1

[commit-bot: I haz the power, 2015-05-29 23:21:37]

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

[commit-bot: I haz the power, 2015-05-29 23:21:38]

Dry run: This issue passed the CQ dry run.

[Sam Clegg, 2015-06-01 21:17:12]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-06-01 21:19:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1158283003/1

[commit-bot: I haz the power, 2015-06-01 23:44:37]

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

[commit-bot: I haz the power, 2015-06-01 23:44:38]

Try jobs failed on following builders:
  linux_chromium_chromeos_ozone_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Sam Clegg, 2015-06-02 00:10:14]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-06-02 00:18:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1158283003/1

[cpu_(ooo_6.6-7.5), 2015-06-02 01:08:25]

rs lgtm

[commit-bot: I haz the power, 2015-06-02 01:39:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-02 01:40:10]

Patchset 1 (id:??) landed as
https://crrev.com/fa36d7041817ddd299c89ca5ecef432c9210fda9
Cr-Commit-Position: refs/heads/master@{#332311}