[heap] Fix undefined shift in LiveObjectRange::AdvanceToNextValidObject.

Bug: 
Change-Id: Ide7fab96e5ba1650aa17ba266e6ed2ed893208d7
Reviewed-on: https://chromium-review.googlesource.com/776658Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49446}

src/heap/mark-compact-inl.h

@@ -408,7 +408,7 @@ void LiveObjectRange<mode>::iterator::AdvanceToNextValidObject() {
       // Clear the first bit of the found object..
       current_cell_ &= ~(1u << trailing_zeros);
 
-      uint32_t second_bit_index = 1u << (trailing_zeros + 1);
+      uint32_t second_bit_index = 0;
       if (trailing_zeros >= Bitmap::kBitIndexMask) {
         second_bit_index = 0x1;
         // The overlapping case; there has to exist a cell after the current
@@ -423,6 +423,8 @@ void LiveObjectRange<mode>::iterator::AdvanceToNextValidObject() {
         }
         cell_base_ = it_.CurrentCellBase();
         current_cell_ = *it_.CurrentCell();
+      } else {
+        second_bit_index = 1u << (trailing_zeros + 1);
       }
 
       Map* map = nullptr;