Change maxDecodedSize to maxDecodedBytes

Based on discussion we decided to use size_t instead of IntSize to
specify the maximum decoded size.

Updated the downsampling algorithm in JPEGImageDecoder.

Added Platform::maxImageDecodedSize() to allow the client to specify
the parameter.

BUG=276666
TEST=JPEGImageDecoderTest.*

Review URL: https://chromiumcodereview.appspot.com/23637015

git-svn-id: svn://svn.chromium.org/blink/trunk@158282 bbb929c8-8fbe-4397-9dbb-9b2b20218538