[utils] Make Abs return an unsigned value

In most cases, this does not matter, especially if assigning to the
source variable again:
x = Abs(x)

But there are cases where it matters, e.g. when being used as argument
to a template function:
DCHECK_EQ(x, Abs(x));
which would currently *not* fail for x==kMinInt.

R=tebbi@chromium.org

Change-Id: Ia5abfe164db602b80a34548e0bf9b22033b77c6e
Reviewed-on: https://chromium-review.googlesource.com/568028Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46586}

src/utils.h

@@ -218,12 +218,12 @@ T JSMin(T x, T y) {
 }
 
 // Returns the absolute value of its argument.
-template <typename T>
-T Abs(T a) {
+template <typename T,
+          typename = typename std::enable_if<std::is_integral<T>::value>::type>
+typename std::make_unsigned<T>::type Abs(T a) {
   return a < 0 ? -a : a;
 }
 
-
 // Floor(-0.0) == 0.0
 inline double Floor(double x) {
 #if V8_CC_MSVC