adb: fix zero-initialization in Block.

Iccfe3bd4fe45a0319bd9f23b8cbff4c7070c9f4d changed Block from using
malloc to std::make_unique, which does the equivalent of
`new char[size]()`, which value initializes the array members to 0.
Switch to `reset(new char[size])` to avoid this costly initialization.

Test: adb_benchmark
Change-Id: I09aacb949a7bd4a946ce35a8ee65d1f451577b72

adb/types.h

@@ -108,7 +108,10 @@ struct Block {
         CHECK_EQ(0ULL, capacity_);
         CHECK_EQ(0ULL, size_);
         if (size != 0) {
-            data_ = std::make_unique<char[]>(size);
+            // This isn't std::make_unique because that's equivalent to `new char[size]()`, which
+            // value-initializes the array instead of leaving it uninitialized. As an optimization,
+            // call new without parentheses to avoid this costly initialization.
+            data_.reset(new char[size]);
             capacity_ = size;
             size_ = size;
         }