Parse and use extra HTTP headers when downloading the payload.

Android OTA backend requires to pass an Authorization HTTP header in
order to download some payload. This patch allows to specify such
header when initiating a payload download from Android.

Bug: 27047110
TEST=Added unittests to check the headers sent.

Change-Id: Iece7e0ee252349bbaa9fb8545da2c34d2a76ae69
diff --git a/common/constants.cc b/common/constants.cc
index fc6df37..f138ce3 100644
--- a/common/constants.cc
+++ b/common/constants.cc
@@ -94,5 +94,7 @@
 const char kPayloadPropertyFileHash[] = "FILE_HASH";
 const char kPayloadPropertyMetadataSize[] = "METADATA_SIZE";
 const char kPayloadPropertyMetadataHash[] = "METADATA_HASH";
+const char kPayloadPropertyAuthorization[] = "AUTHORIZATION";
+const char kPayloadPropertyUserAgent[] = "USER_AGENT";
 
 }  // namespace chromeos_update_engine