Fix media::MovingAverage::count return type

media::MovingAverage::count_ is declared as uint64_t and its getter,
count(), returns size_t.

During the Onion soup'ing of some classes [1] that include this
header (soon) from Blink, some compilers face issues [2].

[1] https://crrev.com/c/1698170/
[2] https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8908195303894439840/+/steps/compile__with_patch_/0/stdout

BUG=704136
R=dalecurtis@chromium.org

Change-Id: I87773edec09f8ae95d6c099e12801148b128c218
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1698422Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#676940}

media/base/moving_average.h

@@ -37,7 +37,7 @@ class MEDIA_EXPORT MovingAverage {
   // Resets the state of the class to its initial post-construction state.
   void Reset();
 
-  size_t count() const { return count_; }
+  uint64_t count() const { return count_; }
 
   base::TimeDelta max() const { return max_; }