Simplify WTF::base64Decode() after r154538

r154538 increased the complexity of WTF::base64Decode() by introducing
a new Base64PaddingValidationPolicy parameter. This patch simplifies
the code a bit while achieving the same functionality and matching
the corresponding WebKit implementation:
http://trac.webkit.org/changeset/153904

Strictly validating padding does not make much sense unless we also
validate strictly invalid characters. Therefore, this patch replaces
the Base64PaddingValidationPolicy enum with an additional
Base64FailOnInvalidCharacterOrExcessPadding value in the
Base64DecodePolicy enum. The code is also improved to validate
the padding at the end of the decoding loop and also removes a
redundant check for the (dataLength % 4 == 1) case.

BUG=261200

Review URL: https://chromiumcodereview.appspot.com/22796004

git-svn-id: svn://svn.chromium.org/blink/trunk@155999 bbb929c8-8fbe-4397-9dbb-9b2b20218538