SkPDF: speed up SkPDFShader generation.

SkPDF: speed up SkPDFShader generation.

Stop using SkString::append() when SkDynamicMemoryWStream
works better.

Also add a bench to prove that this speeds things up:
    before:
        micros   	bench
         59.33 ?	PDFShader	nonrendering
    after:
        micros   	bench
         34.55 ?	PDFShader	nonrendering
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1833793002

Committed: https://skia.googlesource.com/skia/+/d11c7268a2a4303e842f4ba16da98fcd0a2b7555

[hal.canary, 2016-03-24 22:24:20]

Description was changed from

==========
SkPDF: speed up SkPDFShader generation.

Stop using SkString::append() when SkDynamicMemoryWStream
works better.

Also add a bench to prove that this speeds things up:
    before:
        micros   	bench
         59.33 ?	PDFShader	nonrendering
    after:
        micros   	bench
         34.55 ?	PDFShader	nonrendering
==========

to

==========
SkPDF: speed up SkPDFShader generation.

Stop using SkString::append() when SkDynamicMemoryWStream
works better.

Also add a bench to prove that this speeds things up:
    before:
        micros   	bench
         59.33 ?	PDFShader	nonrendering
    after:
        micros   	bench
         34.55 ?	PDFShader	nonrendering
GOLD_TRYBOT_URL=
https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&is...
==========

[hal.canary, 2016-03-24 22:24:44]

halcanary@google.com changed reviewers:
+ caryclark@google.com

[hal.canary, 2016-03-24 22:24:44]

ptal

[caryclark, 2016-03-25 12:15:08]

lgtm

[hal.canary, 2016-03-25 12:38:31]

The CQ bit was checked by halcanary@google.com

[commit-bot: I haz the power, 2016-03-25 12:38:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1833793002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1833793002/1

[commit-bot: I haz the power, 2016-03-25 12:53:00]

Description was changed from

==========
SkPDF: speed up SkPDFShader generation.

Stop using SkString::append() when SkDynamicMemoryWStream
works better.

Also add a bench to prove that this speeds things up:
    before:
        micros   	bench
         59.33 ?	PDFShader	nonrendering
    after:
        micros   	bench
         34.55 ?	PDFShader	nonrendering
GOLD_TRYBOT_URL=
https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&is...
==========

to

==========
SkPDF: speed up SkPDFShader generation.

Stop using SkString::append() when SkDynamicMemoryWStream
works better.

Also add a bench to prove that this speeds things up:
    before:
        micros   	bench
         59.33 ?	PDFShader	nonrendering
    after:
        micros   	bench
         34.55 ?	PDFShader	nonrendering
GOLD_TRYBOT_URL=
https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&is...

Committed:
https://skia.googlesource.com/skia/+/d11c7268a2a4303e842f4ba16da98fcd0a2b7555
==========

[commit-bot: I haz the power, 2016-03-25 12:53:01]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/d11c7268a2a4303e842f4ba16da98fcd0a2b7555