PartitionSizes -> PartitionMetadata

In a following CL, InitPartiitonMetadata not only
takes sizes but also group of each partition. Rename
the struct to a more generic name.

Test: builds
Change-Id: Ie8a7e58b0cccc45e533de368e24f5a73d556de65
diff --git a/boot_control_chromeos.cc b/boot_control_chromeos.cc
index 68ac5bd..a82efc7 100644
--- a/boot_control_chromeos.cc
+++ b/boot_control_chromeos.cc
@@ -303,7 +303,7 @@
 }
 
 bool BootControlChromeOS::InitPartitionMetadata(
-    Slot slot, const PartitionSizes& partition_sizes) {
+    Slot slot, const PartitionMetadata& partition_metadata) {
   return true;
 }