crash-reporter: Create a standalone ebuild for system_api

system_api is required by a lot of platform2 packages. We need it to be in a
standalone ebuild in order for other packages to depend on it.

This package installs the common headers and compiles the protobufs into static
libraries.

BUG=chromium:386223
TEST=FEATURES=test emerge-amd64-generic platform2.
TEST=trybot run on daisy, link, duck, x86-mario.
TEST=trybot run on lumpy-incremental-release.

CQ-DEPEND=CL:204594

Change-Id: I235b5baf03a4497352f7d53ab68326ddb4e9676a
Reviewed-on: https://chromium-review.googlesource.com/204581
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>

crash-reporter.gyp

diff --git a/crash-reporter.gyp b/crash-reporter.gyp
index dfa95ec..c3135b9 100644
--- a/crash-reporter.gyp
+++ b/crash-reporter.gyp
@@ -35,9 +35,6 @@
           ],
         },
       },
-      'dependencies': [
-        '<(platform_root)/system_api/system_api.gyp:system_api-headers',
-      ],
       'sources': [
         'chrome_collector.cc',
         'crash_collector.cc',