Use V8_NOEXCEPT consistently between decls and impls

noexcept keyword needs to be consistent between the declaration and
implementation in C++17.

Bug: v8:8616, chromium:752720
Change-Id: Iff4022c8c4b861ebdbe8e08995af1bc4da866dae
Reviewed-on: https://chromium-review.googlesource.com/c/1396459Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58609}

src/heap/heap.cc

@@ -108,7 +108,7 @@ bool Heap::GCCallbackTuple::operator==(
 }
 
 Heap::GCCallbackTuple& Heap::GCCallbackTuple::operator=(
-    const Heap::GCCallbackTuple& other) = default;
+    const Heap::GCCallbackTuple& other) V8_NOEXCEPT = default;
 
 struct Heap::StrongRootsList {
   FullObjectSlot start;

src/heap/spaces.cc

@@ -2215,9 +2215,8 @@ LocalAllocationBuffer::LocalAllocationBuffer(
   *this = other;
 }
 
-
 LocalAllocationBuffer& LocalAllocationBuffer::operator=(
-    const LocalAllocationBuffer& other) {
+    const LocalAllocationBuffer& other) V8_NOEXCEPT {
   Close();
   heap_ = other.heap_;
   allocation_info_ = other.allocation_info_;