AutoFill: Release the cached frame when we receive the frameDestroyed() message
from WebKit.

BUG=48857
TEST=none

Review URL: http://codereview.chromium.org/3173005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@55789 0039d316-1c4b-4281-b951-d872f2087c98
static double CalculateBoringScore(SkBitmap* bitmap) {
  int histogram[256] = {0};
  color_utils::BuildLumaHistogram(bitmap, histogram);

  int color_count = *std::max_element(histogram, histogram + 256);
  int pixel_count = bitmap->width() * bitmap->height();
  return static_cast<double>(color_count) / pixel_count;
}

static double CalculateBoringScore(SkBitmap* bitmap) {
  int histogram[256] = {0};
  color_utils::BuildLumaHistogram(bitmap, histogram);

  int color_count = *std::max_element(histogram, histogram + 256);
  int pixel_count = bitmap->width() * bitmap->height();
  return static_cast<double>(color_count) / pixel_count;
}
