Added CSSPropertyBackgroundComponentUtils which holds background component parsing methods.

Added CSSPropertyBackgroundComponentUtils which holds background component parsing methods.

Moved methods with background component parsing logic into a utils
file, so that the parsing logic for CSSPropertyBackgroundAttachment,
CSSPropertyBackgroundBlendMode, CSSPropertyBackgroundClip,
CSSPropertyBackgroundImage, CSSPropertyBackgroundOrigin,
CSSPropertyBackgroundPositionX, CSSPropertyBackgroundPositionY,
CSSPropertyBackgroundSize, CSSPropertyMaskSourceType,
CSSPropertyWebkitBackgroundClip, CSSPropertyWebkitBackgroundOrigin,
CSSPropertyWebkitMaskClip, CSSPropertyWebkitMaskComposite,
CSSPropertyWebkitMaskImage, CSSPropertyWebkitMaskOrigin,
CSSPropertyWebkitMaskPositionX, CSSPropertyWebkitMaskPositionY and
CSSPropertyWebkitMaskSize can be moved out of
CSSPropertyParser.cpp into implementations of CSSPropertyAPI.h.

Follow up patches implement CSSPropertyAPI for all the properties.

This patch:
- Adds CSSPropertyBackgroundComponentUtils (.h and .cpp files) to
  the BUILD.gn file.
- Moves consumeBackgroundBlendMode(), consumeBackgroundAttachment(),
  consumeBackgroundBox(), consumeBackgroundComposite(),
  consumeMaskSourceType(), consumePrefixedBackgroundBox(),
  consumeBackgroundSize(), addBackgroundValue() and
  consumeBackgroundComponent() from CSSPropertyParser.cpp to
  CSSPropertyBackgroundComponentUtils.cpp.
- Changes callsites of these methods in CSSPropertyParser.cpp to call
  the methods from CSSPropertyBackgroundComponentUtils.

BUG=668012

[nainar, 2017-01-19 06:15:59]

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

[commit-bot: I haz the power, 2017-01-19 06:16:16]

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

[nainar, 2017-01-19 06:16:33]

nainar@chromium.org changed reviewers:
+ aazzam@google.com

[nainar, 2017-01-19 06:16:34]

Anna, 

PTAL? Thanks!

[commit-bot: I haz the power, 2017-01-19 06:18:20]

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

[commit-bot: I haz the power, 2017-01-19 06:18:21]

Dry run: Try jobs failed on following builders:
  ios-device on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device/builds...)
  ios-device-xcode-clang on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device-xcode-...)
  ios-simulator on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator/bui...)
  ios-simulator-xcode-clang on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator-xco...)

[nainar, 2017-01-20 07:31:32]

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

[commit-bot: I haz the power, 2017-01-20 07:31:51]

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, 2017-01-20 09:05:50]

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

[commit-bot: I haz the power, 2017-01-20 09:05:51]

Dry run: This issue passed the CQ dry run.