webtry: Only create sqlite3 tables if they don't exist yet.

webtry: Only create sqlite3 tables if they don't exist yet.

Otherwise it will through an error like the following:

2014/09/28 21:22:24 Info: status creating sqlite table for sources:
"table source_images already exists"
2014/09/28 21:22:24 Info: status creating sqlite table for webtry:
"table webtry already exists"
2014/09/28 21:22:24 Info: status creating sqlite table for workspace:
"table workspace already exists"
2014/09/28 21:22:24 Info: status creating sqlite table for workspace
try: "table workspacetry already exists"

To test locally the following was done:

$ ./gyp_skia gyp/webtry.gyp gyp/most.gyp -Dskia_gpu=0
$ ninja -C out/Debug webtry
$ cd experimental/webtry
$ go get -d
$ go build webtry.go
$ ./webtry
$ google-chrome http://localhost:8000

Expected: see no more the above messages.

BUG=None
TEST=see above
R=stephana@google.com, jcgregorio@google.com

Author: tfarina@chromium.org

Review URL: https://codereview.chromium.org/613593002