Avoid one temporary copy on web fonts decoding

Before this CL, we allocated a temporary SharedBuffer to store decoded
font data. Since the decoded data is only used to create SkTypeface we don't
need to have such abstraction. Create SkTypeface directly from the decoded
data. This CL also renamed and moved platform/fonts/opentype/OpenTypeSanitizer
to platform/fonts/WebFontDecoder for better naming.

BUG=616969

Review-Url: https://codereview.chromium.org/2032943002
Cr-Commit-Position: refs/heads/master@{#397648}