Replace subpixel font size heuristics with using OpenType gasp table

Replace subpixel font size heuristics with using OpenType gasp table

Respect the gasp table settings in fonts for smoothing and DirectWrite
rendering mode, i.e. activating rendering mode
DWRITE_RENDERING_MODE_CLEARTYPE_NATURAL_SYMMETRIC when the gasp table
instructs to do so.

This should allow us to remove the custom minSizeForSubpixel and
minSizeForAntiAlias cut-off thresholds and rendering mode overrides.

Multi-layer GIMP image files attached to the bug help illustrate the
rendering differences between Chrome stable and Chrome with
gasp table controlled rendering. In these files, significant
readability improvements especially with regards to Simsun and PMingLiu
fonts at low font sizes can be observed.

Thanks to Ben Wagner for the enabling Skia fix in
https://skia-review.googlesource.com/c/8268/

BUG=645055

[drott, 2017-03-06 15:06:07]

The CQ bit was checked by drott@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-03-06 15:06:19]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[drott, 2017-03-06 15:34:39]

drott@chromium.org changed reviewers:
+ bungeman@chromium.org, eae@chromium.org

[commit-bot: I haz the power, 2017-03-06 18:16:39]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-03-06 18:16:40]

Dry run: Try jobs failed on following builders:
  win_chromium_rel_ng on master.tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[eae, 2017-03-07 00:07:43]

This is awesome, the new results look a lot better and I'm glad we can finally
retire the small-font-size hack!

Thank you both!

LGTM

[drott, 2017-03-15 20:45:29]

The CQ bit was checked by drott@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-03-15 20:46:04]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[drott, 2017-03-15 20:47:09]

The CQ bit was unchecked by drott@chromium.org

[drott, 2017-03-16 11:16:11]

The CQ bit was checked by drott@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-03-16 11:16:35]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-03-16 17:53:19]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-03-16 17:53:21]

Dry run: Try jobs failed on following builders:
  win_chromium_rel_ng on master.tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[drott, 2017-03-17 09:41:59]

The CQ bit was checked by drott@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-03-17 09:42:10]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[drott, 2017-03-17 09:48:55]

I think I have the flag logic right now, waiting for another round of bot
results. Previously we did enable subpixel for tests, even though antialiasing
was off for everything other than fast/text. Once we remove the ts >=
m_minSizeForAntiAlias and ts >= m_minSizeForSubpixel tests, this leads to uneven
spacing in all SVG tests for example, because these minSizes were hiding the
spacing problem. We should not activate subpixel positioning without
antialiasing, whether that's in layout tests or otherwise.

[drott, 2017-03-17 15:14:00]

The CQ bit was unchecked by drott@chromium.org

[eae, 2017-03-17 16:53:16]

LGTM

https://codereview.chromium.org/2737533002/diff/40001/third_party/WebKit/Sour...
File third_party/WebKit/Source/platform/fonts/win/FontPlatformDataWin.cpp
(right):

https://codereview.chromium.org/2737533002/diff/40001/third_party/WebKit/Sour...
third_party/WebKit/Source/platform/fonts/win/FontPlatformDataWin.cpp:64: //
Subpixel text positioning leads to uneven spacing without font
This explanation is a little confusing at first as only describes a problem and
not the solution. How about starting with "Only use sub-pixel positioning if
anti aliasing is enabled."?

[drott, 2017-03-24 13:21:01]

>
https://codereview.chromium.org/2737533002/diff/40001/third_party/WebKit/Sour...
> third_party/WebKit/Source/platform/fonts/win/FontPlatformDataWin.cpp:64: //
Subpixel text positioning leads to uneven spacing without font
> This explanation is a little confusing at first as only describes a problem
and not the solution. How about starting with "Only use sub-pixel positioning if
anti aliasing is enabled."?

Addressed in: https://chromium-review.googlesource.com/c/458377/ and moved the
CL there.