[Extensions] Add tests for loading extension with multiple entry points

This test verifies that the extension is loaded even if the entry point
is in a different object than the one implicitly created by the extension
name.
[Extensions] Make entry points info available to the ModuleSystem

If we want to support loading extensions on demand, we have to make the
information for which entry points each extension will be accessible to
the entity responsible for loading extensions, in our case, the entity
is XWalkModuleSystem.
[Extensions] Add a way to tag extensions that will be loaded on demand

For now we will use a simple algorithm that will only load "leaf"
extensions on demand, for example, if "tizen", "tizen.time" and
"tizen.power" are in the extension list, we will enable loading on
demand for "tizen.time" and "tizen.power".
[Extensions] Load extensions on demand

Now that we have all the pieces it is time to put them together.

For each extension marked as 'should use the trampoline', we setup
"trap" acessors (for all entry points), that when run will load the
extension and create an instance for it. Before the extension is
loaded we need to remove all the references to the accessor function,
so it is not run anymore for those objects.
[Add] Add new APIs to XWalkClient for renderer responsiveness

This fix is to add 2 new APIs to XWalkClient for renderer
responsiveness. Internally, xwalk will start a timer to check if the
renderer become unresponsive if no ACK message for handling input event
is received from the renderer in 30 seconds. In this case, the callback
onRendererUnresponsive will be called. If it becomes responsive again,
the callback onRendererResponsive will be called.

BUG=https://github.com/crosswalk-project/crosswalk/issues/913
TEST=org.xwalk.core.xwview.test.RendererResponsivenessTest
Merge pull request #914 from hmin/unresponsive-handler

[Add] Add new APIs to XWalkClient for renderer responsiveness
Merge pull request #967 from vcgomes/load-on-demand

[Extensions] Load extensions on demand
[Extensions] Fix a test to also pass with load on demand

The test was "too strict" and covering only the "expected
failure" (greater than comparison) case. Now that we can run Crosswalk
loading extensions on demand, we can pass it properly (equal
comparison).

Once we enable the feature by default in Crosswalk we should make the
test "strict" again, covering only the equal comparison.
Merge pull request #974 from cmarcelo/fix-test-for-load-on-demand

[Extensions] Fix a test to also pass with load on demand
[Extensions] Flip flag to enable loading extensions on demand

Now that it works, we can flip the flag to be enabled by default.
Merge pull request #976 from vcgomes/flip-load-on-demand-flag

[Extensions] Flip flag to enable loading extensions on demand
Bump version to 2.31.24.0
Merge branch 'master' of github.com-crosswalk:crosswalk-project/crosswalk
[Extension] Fix build failure on Windows

Use '\' for the product_dir on Windows in external_extension_sample.gyp.
Otherwise, the build system can't handle the path well in some cases,
e.g. creating or deleting them.
It fails the windows build when it starts from zero.