[Zucchini]: Change |kInvalidOffset| in image_utils.h

In preparation for a large upcoming DEX support CL |kInvalidOffset|
needs to change as the Dalvik standard uses 0xFFFFFFF*F*U as a sentinel
value. Instead use 0xFFFFFFF*E* to indicate |kInvalidOffset|.

Change-Id: I8ada5beb8e0f5bef28dadb8c6401f95665670b91
Reviewed-on: https://chromium-review.googlesource.com/978380Reviewed-by: Samuel Huang <huangs@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@google.com>
Cr-Commit-Position: refs/heads/master@{#546120}

components/zucchini/image_utils.h

@@ -20,7 +20,8 @@ namespace zucchini {
 using offset_t = uint32_t;
 // Divide by 2 since label marking uses the most significant bit.
 constexpr offset_t kOffsetBound = static_cast<offset_t>(-1) / 2;
-constexpr offset_t kInvalidOffset = static_cast<offset_t>(-1);
+// Use 0xFFFFFFF*E*, since 0xFFFFFFF*F* is a sentinel value for Dex references.
+constexpr offset_t kInvalidOffset = static_cast<offset_t>(-2);
 
 // key_t is used to identify an offset in a table.
 using key_t = uint32_t;