Remove the dependency of platform.gyp to crash-reporter

We are creating a standalone ebuild for crash-reporter. platform.gyp should not
depend on crash-reporter.gyp as crash-reporter will be build and installed by
chromeos-base/crash-reporter.

BUG=chromium:391999
TEST=FEATURES=test emerge-amd64-generic platform2.
TEST=FEATURES=test emerge-amd64-generic crash-reporter.

CQ-DEPEND=CL:206918

Change-Id: I726ed0c847e361b7f03c2ef82fbb8410f40b6c80
Reviewed-on: https://chromium-review.googlesource.com/206919


Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>