Fix uninitialized memory bug in the SkGlyphCache.

The core of the problem is that the system is asked to lookup the metrics for a character with id == 0. This causes a hit in the fCharToGlyphHash matching the sentinel glyph. This happens because fCharToGlpyhHash is initialized with all zeros, therefore, the fID is zero matching the char with id == 0. The fAdvanceX field of the sentinel glyph is in fact not initialized.

The bigger question is now did a zero character get passed to getUnicharMetrics?

The breaking code is basically as follows:
wchar_t glyph = L'S';
paint.measureText(&glyph, 2);

This get mischaracterized as a utf8 string instead of a utf16(?) string. Because of the little endian ordering, this is the character string 'L' '\0'. Since the size of the original string is two bytes (but a single character) the '\0' is treated as its own character and past to getUnicharMetrics.

TEST:
On windows failed using DrMemory. With this change does not fail.

BUG=463204

Review URL: https://codereview.chromium.org/977063002

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