SkPDF: Glyph validation change

Instead of mapping invaid glyphIDs to zero or maxGlyphID,
don't draw them at all.

Validate glyphs when glyph is written, not ahead of time.

Don't allocate array to copy user-provided glyphs.

Easy early exit from SkPDFDevice::internalDrawText()
    GlyphPositioner::flush() called ~GlyphPositioner()
    SkScopeExit class now exists.

Assume SkTypeface* pointers are now never null in more
places.

precalculate alignmentFactor to clean up code.

SkPDFDevice::updateFont must be called with validated
glyphID.  Skip bad glyphs to make this true.

SkPDFDevice::updateFont always succeeds.

SkPDFFont::GetFontResource always succeeds (preconditions are
asserted).  If GetMetrics fails, don't call GetFontResource.

SkPDFFont::glyphsToPDFFontEncodingCount() becomes
SkPDFFont::countStretch() and is inlined.

SkPDFFont::glyphsToPDFFontEncoding now works one Glyph at a
time and is inlined.

SkPDFFont::noteGlyphUsage() operates one glyph at a time.

Add SkScopeExit.h; also a unit test for it.

SkPostConfig: Fix SK_UNUSED for Win32.

No public API changes.
TBR=reed@google.com

BUG=625995

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2278703002

Review-Url: https://codereview.chromium.org/2278703002
7 files changed