[Tizen] Add unit test for metadata configuration element handler

This patch adds tizen_metadata_handler_unittest.cc. Now test verify
that the handler properly handles the following configuration elements:
widget.metadata @key @value
[Tizen] xwalk crashed when requests "json/new" in devtools

The main reason is:
The intialization of "XWalkContentBrowserClient::browser_context_"
depends on invoking "XWalkContentBrowserClient::CreateRequestContext",
so in this case when "GetDevToolsManagerDelegate" is creating, the
"browser_context_" is still NULL. And we can init "browser_context_"
through "xwalk_runner_" in constructor function to make sure it's valid
at least anytime.

BUG=https://crosswalk-project.org/jira/browse/XWALK-3178
Speed up GetDefaultWidgetEntryPage function

This patch introduces three improvements in the function looking
for widget startup page. There were unnecessary tests when the
result was already found.
'WidgetHandlerTest' test case extension

Add validation for manifest and widget preferences for:
WidgetHandlerTest.ParseManifestWithAllOfOtherItemsAndOnePreferenceItem
test case.
[Tizen] Fix memory leaks found in xwalk_tizen_user.cc

Memory leaks appear when xwalk_backend runs correctly.
This patch adds freeing buffer after usage.

BUG=XWALK-3206
Add the check for return value of GetString().

This patch is to add the check for return value of GetString() in
widget_handler.cc and xwalk_content.cc.

CID=220010

Related to XWALK-2928.
Merge pull request #2725 from wuyuehang/XWALK-3187

[Tizen] xwalk crashed when requests "json/new" in devtools
Merge pull request #2739 from KFilipek/XWALK-3206

[Tizen] Fix memory leaks found in xwalk_tizen_user.cc
Merge pull request #2727 from aszafranek/WidgetHandlerTestExtension

'WidgetHandlerTest' test case extension
Merge pull request #2703 from KFilipek/AddMetadataUnitTest

[Tizen] Add unit test for metadata configuration element handler
Merge pull request #2744 from tweglarski/GetDefaultWidgetEntryPageSpeedUp

Speed up GetDefaultWidgetEntryPage function
Bump version to 11.40.268.0
[Runtime] Add the initialization for RuntimeFeature.

The struct about RuntimeFeature was not initialized with empty constructor,
assign the constructor with "default" to initialize the member variables.

CID=194651

Related to XWALK-2928.
Merge pull request #2697 from wuhengzhi/xwalk_2928_cid194651_new

[Runtime] Add the initialization for RuntimeFeature.
Remove validation on uri field of app-control

Following the documentation, uri could
- point a ressource without defining a sheme
- be empty

so remove the uri check
BUG=XWALK-3223
Signed-off-by: Baptiste DURAND <baptiste.durand@open.eurogiciel.org>
Bump version to 11.40.269.0
Merge pull request #2750 from eurogiciel-oss/XWALK-3223

Remove validation on uri field of app-control
Bump version to 11.40.270.0
Merge pull request #2694 from wuhengzhi/xwalk_2928_cid220010

Add the check for return value of GetString().
Bump version to 11.40.271.0