We seem to want to have a 10MiB quota for localstorage [1]. But only around 5MiB is allowed according to various webby tests like [2].

I think the problem is that for accounting we multiply by 2 the length of the strings *in bytes* (as specified by std::string spec) in [3].

Please double-check, I am not 100% confident in my knowledge of the code.

[1] kPerStorageAreaQuota is 10MiB
    https://codesearch.chromium.org/chromium/src/content/common/dom_storage/dom_storage_types.h?q=kPerStorageAreaQuota&l=24

[2] localstorage test
    https://arty.name/localstorage.html

[3] definition of size_of_item() in dom_storage_map
    https://codesearch.chromium.org/chromium/src/content/common/dom_storage/dom_storage_map.cc?type=cs&q=size_of_item+package:%5Echromium$&l=13

[4] announcement of quota increase
    https://plus.google.com/u/0/+FrancoisBeaufort/posts/S5Q9HqDB8bh