Fix premature termination of scroll animations while selecting text.

Fix premature termination of scroll animations while selecting text.

During text selection, AutoscrollController calls scrollIntoView continuously
to make the scroll position follow the selection.  This had the side effect of
resetting ScrollAnimator before it could finish a main-thread smooth scroll.

There are early returns for zero-delta scrolls in FrameView::setScrollPosition
and RootFrameViewport::distributeScrollBetweenViewports, but scrollIntoView's
conversion of fractional scroll offsets to LayoutUnits resulted in a very small
(< 1px) non-zero scroll delta.

We now avoid this by directly comparing the input and output of getRectToExpose
and skipping the unneeded calls to setScrollPosition.

BUG=591918
Review URL: https://codereview.chromium.org/1775213002

Cr-Commit-Position: refs/heads/master@{#380162}
(cherry picked from commit 67a7b9d7646d67829d2991376cef8462ff4c6071)

Committed: https://chromium.googlesource.com/chromium/src/+/e06e96e69b17195071fd80772f331e957afd9465

[skobes, 2016-03-14 17:44:18]

Description was changed from

==========
Fix premature termination of scroll animations while selecting text.

During text selection, AutoscrollController calls scrollIntoView continuously
to make the scroll position follow the selection.  This had the side effect of
resetting ScrollAnimator before it could finish a main-thread smooth scroll.

There are early returns for zero-delta scrolls in FrameView::setScrollPosition
and RootFrameViewport::distributeScrollBetweenViewports, but scrollIntoView's
conversion of fractional scroll offsets to LayoutUnits resulted in a very small
(< 1px) non-zero scroll delta.

We now avoid this by directly comparing the input and output of getRectToExpose
and skipping the unneeded calls to setScrollPosition.

BUG=591918

Review URL: https://codereview.chromium.org/1775213002

Cr-Commit-Position: refs/heads/master@{#380162}
(cherry picked from commit 67a7b9d7646d67829d2991376cef8462ff4c6071)
==========

to

==========
Fix premature termination of scroll animations while selecting text.

During text selection, AutoscrollController calls scrollIntoView continuously
to make the scroll position follow the selection.  This had the side effect of
resetting ScrollAnimator before it could finish a main-thread smooth scroll.

There are early returns for zero-delta scrolls in FrameView::setScrollPosition
and RootFrameViewport::distributeScrollBetweenViewports, but scrollIntoView's
conversion of fractional scroll offsets to LayoutUnits resulted in a very small
(< 1px) non-zero scroll delta.

We now avoid this by directly comparing the input and output of getRectToExpose
and skipping the unneeded calls to setScrollPosition.

BUG=591918

Review URL: https://codereview.chromium.org/1775213002

Cr-Commit-Position: refs/heads/master@{#380162}
(cherry picked from commit 67a7b9d7646d67829d2991376cef8462ff4c6071)

Committed:
https://chromium.googlesource.com/chromium/src/+/e06e96e69b17195071fd80772f33...
==========

[skobes, 2016-03-14 17:44:20]

Committed patchset #1 (id:1) manually as
e06e96e69b17195071fd80772f331e957afd9465.