update_engine: add chrome os device requisition to the omaha request

Requisition type is not currently sent to Omaha and is thus unavailable
for AU rules. This adds a requisition attribute to the <app> if the
device has a requisition type. Currently, the types may be one of
remora, shark, or rialto.

TEST=unittest
BUG=b:132014633,b:133324571

Change-Id: I0e53d3a5749da4cbb95ce73cff35191066339009
Reviewed-on: https://chromium-review.googlesource.com/1604218
Commit-Ready: Matthew Ziegelbaum <ziegs@chromium.org>
Tested-by: Matthew Ziegelbaum <ziegs@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/hardware_android.h b/hardware_android.h
index c59a152..6edf468 100644
--- a/hardware_android.h
+++ b/hardware_android.h
@@ -42,6 +42,7 @@
   std::string GetHardwareClass() const override;
   std::string GetFirmwareVersion() const override;
   std::string GetECVersion() const override;
+  std::string GetDeviceRequisition() const override;
   int GetMinKernelKeyVersion() const override;
   int GetMinFirmwareKeyVersion() const override;
   int GetMaxFirmwareKeyRollforward() const override;