[CSS Grid Layout] Support sparse in auto-placement algorithm

Sparse mode for auto-placement algorithm was not implemented yet.

This patch implements sparse mode for auto-placement algorithm. It keeps
track of the auto-placement cursor in
RenderGrid::placeAutoMajorAxisItemsOnGrid() and updates it accordingly
when auto-positioned items are placed.
If we're in dense mode it resets the cursor after each item.

GridIterator has been adapted to look for empty areas from a given
position in both directions.

Test cases have been adapted accordingly, adding new cases to cover both
sparse and dense options.

TEST=fast/css-grid-layout/grid-auto-flow-sparse.html

BUG=384099

Review URL: https://codereview.chromium.org/362733002

git-svn-id: svn://svn.chromium.org/blink/trunk@178262 bbb929c8-8fbe-4397-9dbb-9b2b20218538
initialize result variables in v8 bindings

https://codereview.chromium.org/360703003 and
https://codereview.chromium.org/362993004 can falsely trigger
use of uninitialized variable warnings in some toolchains,
for integer "return" variables.  Let's explicitly initialize
these variables.

BUG=341031

Review URL: https://codereview.chromium.org/390223004

git-svn-id: svn://svn.chromium.org/blink/trunk@178263 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Refactor verifyProtocolHandlerScheme() in NavigatorContentUtils.cpp

Remove redundant parameter in this function as well as rename the function name
to verifyCustomHandlerScheme() to be sync with other helper function.

BUG=none

Review URL: https://codereview.chromium.org/398743002

git-svn-id: svn://svn.chromium.org/blink/trunk@178264 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Apply toFoo generation macro to TrackBase classes

As a step to use toFoo, this cl introduces DEFINE_TRACK_TYPE_CASTS, and use it.

BUG=309516

Review URL: https://codereview.chromium.org/392193002

git-svn-id: svn://svn.chromium.org/blink/trunk@178265 bbb929c8-8fbe-4397-9dbb-9b2b20218538
WebAudio: Make the arguments of AudioParam::connect and disconnect references.

We can remove unnecessary null checks safely.

BUG=

Review URL: https://codereview.chromium.org/393023007

git-svn-id: svn://svn.chromium.org/blink/trunk@178266 bbb929c8-8fbe-4397-9dbb-9b2b20218538