[Extensions] Add command line switch xwalk-extension-cmd-prefix

This will prepend the contents of this flag to the command line
when starting extension process, it works as same as the switch
"--renderer-cmd-prefix", could be useful if we need to debug
the extension process at its very beginning.
Useful values might be "valgrind" or "xterm -e gdb --args".

Also modified the child path flag which is used to retrieve the
child process' execution file according to the extension command
line prefix on linux. In most cases we need to use CHILD_ALLOW_SELF
to exec child process from /proc/self/exe to prevent chromium's
autoupdate feature to change the file out from underneath; For the
case of xwalk-extension-cmd-prefix, as we suppose to override the
command line to load the app directly, we need to use CHILD_NORMAL
to exec child process from its real app path.
Bump version to 6.34.116.0
Merge pull request #1780 from jondong/master

[Extensions] Add command line switch xwalk-extension-cmd-prefix
Move to Chromium 35.0.1916.17.

This is another one of those big commits made of a lot of smaller squashed
commits done in our "next branch".

Some of our tests are broken but pass in Release mode; we are working on
fixing them.

Most of the changes in the code are caused by upstream Chromium changes,
namely:

* https://codereview.chromium.org/109013004
* https://codereview.chromium.org/120303003
* https://codereview.chromium.org/126763003
* https://codereview.chromium.org/135013008
* https://codereview.chromium.org/154473002
* https://codereview.chromium.org/159173002
* https://codereview.chromium.org/161113002
* https://codereview.chromium.org/165893002
* https://codereview.chromium.org/172163003
* https://codereview.chromium.org/177863002
* https://codereview.chromium.org/183743021
* https://codereview.chromium.org/183853011
* https://codereview.chromium.org/184563006
* https://codereview.chromium.org/185713008
* https://codereview.chromium.org/185863003
* https://codereview.chromium.org/186883002
* https://codereview.chromium.org/187223003
* https://codereview.chromium.org/187223003
* https://codereview.chromium.org/189333004
* https://codereview.chromium.org/190663012
* https://codereview.chromium.org/190663012
* https://codereview.chromium.org/190853004
* https://codereview.chromium.org/191093002
* https://codereview.chromium.org/196073002
* https://codereview.chromium.org/196413016
* https://codereview.chromium.org/198333004
* https://codereview.chromium.org/199063003
* https://codereview.chromium.org/206713004
* https://codereview.chromium.org/207353003

Additionally, the following commmits are worth mentioning:

  [Tizen] Remove Mesa patch that's not needed anymore.
  This was fixed differently upstream with r255915 in Chromium.

  Sync gyp_xwalk with upstream gyp_chromium
  Synced to revision r258119.

  [Android] Fix core shell crash issue after rebasing to Chromium35
  icu_use_data_file_flag has been enabled on Android after Chromium35.
  Need to package icudtl.dat resource.
  TODO:
  1. It will also be crashed on Shared mode, caused by application context issue.
  2. CoreShell will be crashed when exit. Will track with XWALK-1366.