Fix the issue File Download Crash in Debug mode

Crosswalk hits DCHECK_NE(content::DownloadItem::kInvalidId, id) failed, since
chromium upstream improved the code, we needed to override the virtual function
GetNextId().
Please see: https://codereview.chromium.org/15584002
This CL also fixed FileDownload error in Linux trybots.

https://github.com/crosswalk-project/crosswalk/issues/888
Remove GTK code from xwalk

Since we're always using aura on linux desktop, GTK-related code in xwalk is deprecated and should be removed.
Merge pull request #890 from cmarcelo/nested-test

[Extensions] Test extensions with nested names
Merge pull request #896 from yinli/FileDownload

Fix the issue File Download Crash in Debug mode
Bump version to 2.31.18.0
Merge pull request #899 from pozdnyakov/Remove_GTK

Remove GTK code from xwalk
Fix the issue that getUserMedia can't work on Linux, Tizen and Windows platform

This is a general fix on Linux, Tizen and Windows platform, since the chromium rebasing
resulted in this issue.
XWalkMediaCaptureDevicesDispatcher::RunRequestMediaAccessPermission wasn't running.
Merge pull request #901 from yinli/getUserMedia_Linux

Fix the issue that getUserMdeida can't work on Linux, Tizen and Windows ...
[Extensions] Add --enable-loading-extensions-on-demand switch

Runtime switch to protect the implementation of on demand loading of
extensions ("lazy load") until it's done, when we change the semantics
to be --disable.

In the release this feature debuts, we keep the disable switch to allow
QA disable if needed. After we succesfully ship one stable Crosswalk
with the feature (it will be Crosswalk 2), the current plan is to remove
the switch, because we don't intend to keep the codepath for the
non-lazy behavior.
Merge pull request #909 from cmarcelo/load-on-demand-switch

[Extensions] Add --enable-loading-extensions-on-demand switch
[Android] The first step to fix the resource id generation for core library.

Currently the resource ids for org.chromium.ui and org.chromium.content
are not generated so the web app will crash when running. Because
the container can't get the resources and their ids.
This patch is the first step to copy necessary resources from these two
targets to xwalk core library. The next step is to generate the correct
R.java.

BUG=#862
Merge pull request #906 from yongsheng/862

[Android] The first step to fix the resource id generation for core libr...
Bump version to 2.31.19.0
[Extensions] Load extensions on demand for a subset of extensions
[Extensions] Remove unused function
[Extensions] Use a vector instead of a map for extension modules

In XWalkModuleSystem, use a vector instead of a map for extension
modules. The number of modules is small and we'll want sorted iteration
later to identify the extension modules that will use "trampolines" and
those that won't.
tmp
[Extensions] minor fixes
[Extensions] Add SetExtraEntryPoints to Extension C API

This will allow for Extension to inform if they have other entry
points (apart from the implicit extension_name), that should cause
the extension to be loaded.

For example, the "calendar" extension may export some constants in the
CalendarAppointment object, and we want the calendar extension to be
loaded when any of these "main" objects are accessed.
WIP: Multiple entry points and tests