metrics: Update gyp files to use libchromeos from src/platform2

libchromeos was moved from src/platform to src/platform2. This CL updates
the gyp files to use the new location.

BUG=chromium:370258
TEST=FEATURES=test emerge-amd64-generic platform2
TEST=trybot run on daisy, link, mario and duck

CQ-DEPEND=CL:202334

Change-Id: I939dd5672120d47e8dc388f75b3dfeeb234c47b7
Reviewed-on: https://chromium-review.googlesource.com/202398


Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>