Revert of Fixes for SkRWBuffer

Revert of Fixes for SkRWBuffer (patchset #5 id:80001 of https://codereview.chromium.org/1871953002/ )

Reason for revert:
Making MSAN and TSAN rather unhappy.

https://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-MSAN/builds/1586

https://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN/builds/5922

Original issue's description:
> Fixes for SkRWBuffer
> 
> Do not call SkBufferHead::validate in SkROBuffer's destructor, which
> may be called in a separate thread from SkRWBuffer::append. validate()
> reads SkBufferBlock::fUsed, and append() writes to it, resulting in
> a data race.
> 
> Update some comments to be more clear about how it is safe to use
> these classes across threads.
> 
> Test the readers in separate threads.
> 
> In addition, make sure it is safe to create a reader even when no
> data has been appended. Add tests for this case.
> 
> Mark a parameter to SkBufferHead::validate() as const, reflecting
> its use.
> 
> BUG=chromium:601578
> GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1871953002
> 
> Committed: https://skia.googlesource.com/skia/+/d06920a29fe11c68bde2b93948ec99f277bb8459

TBR=mtklein@google.com,reed@google.com,scroggo@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:601578
Committed: https://skia.googlesource.com/skia/+/a3760992c93ddb5512d96671831576907441605d

[bungeman-skia, 2016-04-14 21:56:39]

Created Revert of Fixes for SkRWBuffer

[commit-bot: I haz the power, 2016-04-14 21:56:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1882853004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1882853004/1

[commit-bot: I haz the power, 2016-04-14 21:57:03]

Description was changed from

==========
Revert of Fixes for SkRWBuffer (patchset #5 id:80001 of
https://codereview.chromium.org/1871953002/ )

Reason for revert:
Making MSAN and TSAN rather unhappy.

https://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX...

https://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX...

Original issue's description:
> Fixes for SkRWBuffer
> 
> Do not call SkBufferHead::validate in SkROBuffer's destructor, which
> may be called in a separate thread from SkRWBuffer::append. validate()
> reads SkBufferBlock::fUsed, and append() writes to it, resulting in
> a data race.
> 
> Update some comments to be more clear about how it is safe to use
> these classes across threads.
> 
> Test the readers in separate threads.
> 
> In addition, make sure it is safe to create a reader even when no
> data has been appended. Add tests for this case.
> 
> Mark a parameter to SkBufferHead::validate() as const, reflecting
> its use.
> 
> BUG=chromium:601578
> GOLD_TRYBOT_URL=
https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&is...
> 
> Committed:
https://skia.googlesource.com/skia/+/d06920a29fe11c68bde2b93948ec99f277bb8459

TBR=mtklein@google.com,reed@google.com,scroggo@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:601578
==========

to

==========
Revert of Fixes for SkRWBuffer (patchset #5 id:80001 of
https://codereview.chromium.org/1871953002/ )

Reason for revert:
Making MSAN and TSAN rather unhappy.

https://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX...

https://build.chromium.org/p/client.skia/builders/Test-Ubuntu-GCC-GCE-CPU-AVX...

Original issue's description:
> Fixes for SkRWBuffer
> 
> Do not call SkBufferHead::validate in SkROBuffer's destructor, which
> may be called in a separate thread from SkRWBuffer::append. validate()
> reads SkBufferBlock::fUsed, and append() writes to it, resulting in
> a data race.
> 
> Update some comments to be more clear about how it is safe to use
> these classes across threads.
> 
> Test the readers in separate threads.
> 
> In addition, make sure it is safe to create a reader even when no
> data has been appended. Add tests for this case.
> 
> Mark a parameter to SkBufferHead::validate() as const, reflecting
> its use.
> 
> BUG=chromium:601578
> GOLD_TRYBOT_URL=
https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&is...
> 
> Committed:
https://skia.googlesource.com/skia/+/d06920a29fe11c68bde2b93948ec99f277bb8459

TBR=mtklein@google.com,reed@google.com,scroggo@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:601578

Committed:
https://skia.googlesource.com/skia/+/a3760992c93ddb5512d96671831576907441605d
==========

[commit-bot: I haz the power, 2016-04-14 21:57:04]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/a3760992c93ddb5512d96671831576907441605d