Fix a TODO.

Technically this variable was not uninitialized, as the caller sets it
to 0 beforehand.  But this is fragile and unclear.

Bug: none
Change-Id: I85b56897e9dd767835efa14fd8389ce3005f8e89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586185Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836228}

base/numerics/checked_math_impl.h

@@ -313,11 +313,11 @@ struct CheckedLshOp<T,
     }
 
     // Handle the legal corner-case of a full-width signed shift of zero.
-    const bool is_valid =
-        std::is_signed<T>::value && !x &&
-        as_unsigned(shift) == as_unsigned(std::numeric_limits<T>::digits);
-    // TODO(pkasting): Doesn't this need to set *result = 0?
-    return is_valid;
+    if (!std::is_signed<T>::value || x ||
+        as_unsigned(shift) != as_unsigned(std::numeric_limits<T>::digits))
+      return false;
+    *result = 0;
+    return true;
   }
 };