ServiceWorker: Read the blob from SW's response to a fetch event

ServiceWorker: Read the blob from SW's response to a fetch event

Also remove browser tests for the fetch event that will be replaced with layout
tests in the next Blink patch. Since layout tests can't test HTTP response
headers, one of the browser tests remains.

Multi-sided patch to implement blob-type fetch event
response bodies:
1: blink-side: https://codereview.chromium.org/307063002/
2: chromium-side, THIS PATCH
3: blink-side: https://codereview.chromium.org/304233017

Originally landed in r274773 and reverted in r274799 for breaking ASAN.
Relanding after fixing a buffer overflow caused by using the wrong
std::string ctor.

BUG=379074
TBR=michaeln

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=274915

[falken, 2014-06-04 14:13:37]

Michael,

I'm relanding this after a simple fix. Patchset 1 is the original
(https://codereview.chromium.org/304153015/) which was reverted due to ASAN
failures. I thought I was using:
  std::string(char*, offset, length)
but it was really:
  std::string(string, offset, length)

[falken, 2014-06-04 14:14:50]

The CQ bit was checked by falken@chromium.org

[commit-bot: I haz the power, 2014-06-04 14:18:34]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/falken@chromium.org/315023003/20001

[michaeln, 2014-06-04 20:15:56]

lgtm

[commit-bot: I haz the power, 2014-06-04 21:52:37]

Change committed as 274915